LD Didactic (LEYBOLD) Scientific Teaching Manual

Transcription

P5.8.6.1 / 2/ 3/ 4/ 5
Diode Pumped Nd:YAG Laser
4747106 EN
Table of Contents
1.0 THE ND:YAG-LASER
4
1.1
Optical pumping
4
1.2
1.2.1
1.2.2
1.2.3
1.2.4
1.2.5
1.2.6
1.2.7
1.2.8
Four-level system of the Nd:YAG laser.
Rate equation model for four levels Spontaneous process
Induced processes
Solution of the rate equations
Steady-state solution Time-dependent solution
Spiking
Q-switching
5
6
6
6
7
7
8
8
8
2.0 LASER RESONATOR
10
2.1
Types of resonators
10
2.2
Stability criterion
10
2.3
2.3.1
2.3.2
2.3.3
Resonator modes
Longitudinal modes
Gain profile
Transverse modes
11
11
12
12
3.0 THE PUMP-LIGHT SOURCE
3.1
Diode Laser
4.0 SECOND HARMONIC GENERATION
13
13
16
4.1
Introduction
16
4.2
Interaction of light and matter
16
4.3
Non-linear Optics
16
4.4
Frequency doubling
17
4.5
Phase matching
17
5.0 Q-SWITCH
21
5.1
Mechanical Q-switch 21
5.2
5.2.1
5.2.2
Optical Q-switch
Passive Q-switch
Pockels’s cell
21
21
22
6.0 SET-UP AND PERFORMANCE OF THE ND:YAG-LASER
25
6.1
The Modules
25
6.2
Experimental set-up
28
6.3
6.3.1
6.3.2
6.3.3
Absorption and wavelength of the laser diode
Adjusting a parallel beam by means of the collimator (module B)
Inserting the focusing unit (module C)
Inserting the YAG - rod
28
28
28
29
6.4
Absorption spectrum 29
6.5
Wavelength and temperature dependence
30
6.6
Laser diode output power
The 4F3/2 life-time measurement
The Nd:YAG laser set-up
30
6.7
6.8
31
32
6.9
Laser resonator adjustment
6.10 Laser operation
6.10.1 Measurement of threshold and output power of the Nd:YAG laser
6.10.2 Output power at various pump wavelengths
6.10.3 Display of transverse modes
6.10.4 Demonstration of Spiking
6.10.5 Operation of the 1.3 µm Line
6.10.6 Mode Aperture and TEM00 Operation
7.0 SECOND HARMONIC GENERATION
7.1
Modules and Set-Up
8.0 PASSIVE Q-SWITCH
8.1
8.1.1
Modules and Set-Up
Experimental arrangement for Q-switch operation
9.0 ACTIVE Q-SWITCH
32
32
32
33
33
33
34
34
35
35
36
36
36
38
9.1
9.1.1
Modules and Set-Up
Description of the modules
38
38
9.2
Active Q-switch alignment procedure 39
9.3
Intra-cavity second harmonic generation with Q-switched laser
40
9.4
Extra-cavity second harmonic generation with Q-switched laser
40
10.0 LASER SAFETY
41
The Nd:YAG Laser: Optical pumping
1.0The Nd:YAG Laser
1.1 Optical pumping
Optical pumping is a process in which light is radiated
into a specimen under investigation and the effect of the
light on the specimen is examined. It was in this way that
the strange physical phenomenon was observed of atoms
only being able to accept or release energy in well defined
quantities. This observation led to the conclusion that atoms only have discrete energy states or energy levels.
When light is absorbed or emitted, a transfer takes place
between the energy levels (Fig. 1.1). A transition from the
level with the energy E1 to a level with the energy E2 can
occur if an incoming photon is absorbed with the energy
EPh = E2 - E1. In the reverse case, a photon with the energy EPh = E2 - E1 is emitted if a transition of the atom
takes place from a state with energy E2 to one with energy
E1. The two processes of absorption and emission differ in
that an external field with the energy EPh must be present
for absorption, whereas no field is present for emission.
This emission occurs spontaneously from the system itself
without external fields. It can be compared to the radioactive decay of an excited nucleus. The analogous inverse
process to absorption, i.e. emission under the application
of external fields is termed induced emission.
uph is the energy density of the external field.
By integration of the equation for spontaneous emission,
information is obtained on the variation of this type of
emission with respect to time:
n 2 (t )
n 2 ( t 0 )⋅ e
A 21⋅t
A decay probability and a mean life-time can be given
completely analogous to radioactive decay. The Einstein
coefficient A 21 represents this probability and
τ=
1
A 21
the mean life-time.
This states the time which passes before the number of excited atoms has reduced to 1/e or before n 2 (t) has reached
the value
n 2 (t )
n 2 ( t 0 )⋅
1
e
For normal optical transitions, this value is between 10-8
and 10-9 sec. This life-time which is determined by the
spontaneous transitions alone is relevant for the natural
half width of a spectral line. According to the Heisenberg
uncertainty principle, there is a relationship between the
width and the life-time:
2π ⋅dν
1
τ
A 21
where dν is the half-width of the spectral line.
E2
Absorption
E1
According to the above requirements transitions will only
take place if the energy of the absorbed or emitted photon
is sharply defined, e. g.
E ph
E2
E1
.
In actual fact nature is not that critical and it is seen that
E2
photons with a slightly different energy also take part in
these processes. The reason for this is that the energy levEmission
els are broadened due to various mechanisms.
Depending on how mobile the atoms are due to their temE1
perature and how they are affected by interactions with
their environment, there is a broadening dE which means
Fig. 1.1: Absorption and emission
that photons within this region are accepted. The width of
For each of the processes the number of atoms can be stat- the transition is given by the half width dE for the relevant
ed which absorb or emit a photon per unit of time and per transition (Fig. 1.2). The same theory is valid for emission.
unit of volume.
B12 ⋅ n1 ⋅ u ph
Absorption
B21 ⋅ n 2 ⋅ u ph
Induced emission
dn 2
= A 21 ⋅ n 2
dt
Spontaneous emission
1
Absorption
dn1
dt
dn 2
dt
dE
B12
denotes the Einstein coefficient of absorption,
0
B21 the Einstein coefficient of induced emission
and
E0
Energy
A21 the Einstein coefficient of spontaneous
Fig. 1.2: Broadened absorption transition.
emission,
n1 and n2 are the densities of the atoms in the states 1 E0 is the energy at which the absorption is the highest. It
corresponds to the value E2- E1. For the sake of completeand 2 respectively,
Dr. Walter Luhs - Jan. 1999, revised 2003, 2009, 2011
Page 4
The Nd:YAG Laser: Four-level system of the Nd:YAG laser
ness it should be mentioned that there are also situations
in which this value can be displaced. The shape of the absorption curve corresponds to a Gaussian distribution. By
definition dE is the width of the curve at which the absorption has fallen to one half of the maximum value.
This is known as the full width at half maximum (FWHM).
If there are other transitions within the vicinity of this transition, they may overlap, producing a substantially wider
absorption curve (Fig. 1.3). This is particularly important
in the case of the absorption of laser diode radiation in
Nd:YAG which is discussed later.
4
F5/2
radiationless transfer
4
804.4 808.4 812.9
817.3
4
I9/2
Absorption
absorption
1064 946
I 13/2
4
4
1322
F3/2
I11/2
emission
Fig. 1.4: Relevant energy levels of Nd:YAG for optical
pumping with laser diodes having wavelengths around 805
nm
Some energy levels of the Nd atom are illustrated in Fig.
1.4. Here, only those are shown which are significant for
optical pumping with laser diodes and which are important for the laser process. The levels are labelled with their
E01
E02
E03
spectroscopic notations. Since the Nd atoms are situated
within the YAG host crystal, the otherwise degenerated
Energy
energy levels of the isolated Nd atom split into a number
Fig. 1.3: Absorption for three neighbouring transitions with of states.
This gives rise to the ground state 4I9/2 from 5 sub-states
different absorption strengths
and the state 4F5/2, which can be pumped from 5 subs-tates.
In principle an atom may have any number of energy lev- Since the wavelength
of the pump-light source (diode laels, but they must be discrete. The transitions between the ser) can vary within low limits, a total of three to four tranindividual levels take place according to defined selection sitions can be pumped with high efficiency.
criteria. When the atom is excited with a defined energy, The Nd atoms of the 4F state pass very quickly into the
an emission spectrum is observed with characteristic ener- laser output level 4F .5/2
3/2 The laser transition which is techgies and this spectrum gives precise information about the nically most interesting
takes place between the 4F3/2 state
energy levels involved.
as starting level and terminates in the 4I11/2 state with an
Excitation by optical pumping has therefore developed as emitted wavelength of 1064 nm.
a very important method used in spectroscopy. It is also an From here the Nd atoms relax again into the ground state
indispensable technique for the excitation of a number of 4l until the pumping process starts from the beginning
9/2
different types of lasers.
again. The Neodymium therefore has an ideal four level
1. Optical pumping in conjunction with Nd:YAG lasystem.
sers is of particular interest, because these have
become widely accepted for industrial use, along
1.2Four-level system of the
with the CO2 laser. The laser-active material which,
in the case of the Nd:YAG laser, is excited by optiNd:YAG laser.
cal pumping, consists of Neodymium atoms that are
The principle is shown in Fig. 1.5. Under the radiation of a
accommodated in a transparent host crystal (YAG =
light field (optical pumping), transitions from ground state
Yttrium Aluminium Garnet).
1 to the upper level 4 occur. The reverse processes from
2.Whereas up to a few years ago Nd:YAG lasers were
state 4 to state 1 are prevented by very fast transitions
almost excited using discharge lamps, optical pumpfrom state 4 to state 3 without radiation. The laser transiing with laser diodes is becoming more significant.
tion takes place from level 3 into level 2 which is thermally
3. This is because laser diodes are available econominot populated. From here the Nd atoms relax again back to
cally and they emit narrow band light at high optiground state 1.
cal powers, which matches the energy levels of the
The irradiation by light, which leads to the population
Nd:YAG crystal (Fig. 1.4). The advantage over the
of an otherwise empty state, is termed optical pumping.
discharge lamp is that the emission of laser diodes is
The emptying of a level occurs either with the emission
nearly completely absorbed by the Nd:YAG, whereas
of photons or without radiation. Transitions without rathe very wide spectral emission of discharge lamps
diation take place due to mechanical interactions such as
is absorbed to only a small extent. The efficiency of
collisions or vibrations and they are also designated as reoptical pumping with discharge lamps is about 3%,
laxation. The number of transitions without radiation per
but figures of up to 50% can be achieved using laser
second is termed the relaxation rate. Transitions in which
diodes!
Page 5
photons are emitted occur spontaneously or are induced. given a negative sign. This is carried out for each of the
Spontaneous transitions also occur without pumping proc- involved states.
esses. However, induced emissions only occur if a pump- The number of excited atoms per unit of time in state 3 is:
ing process takes place. Rates are also stated here, one
dN 3
η ⋅ W14 ⋅ N1 Wp ⋅ N1
pump rate
rate for spontaneous emission and another one for induced
p
dt
emission. Each state which can interact with one or more
other states is labelled with this type of rates.
where η is the pumping efficiency. The transition from
4
state 4 occurs so fast that the level 3 is pumped immediately and the population N4 density of state 4 is therefore
N4∼0.
3
absorption
induced
emission
spont.
Another process affecting state 3 is spontaneous emission:
2
relaxation
1
1.2.2Spontaneous process
Fig. 1.5: Principle of the four-level laser
dN 3
dt
S
Γ ⋅ N3
spontaneous rate,
where Γ=1/τs. τs is the mean life-time of a photon before it
is spontaneously emitted.
1.2.3Induced processes
W14
S43
S32
W32
W23
S21
probability of absorbing a pump photon.
probability of relaxation from state 4 to 3 .
probability of spontaneous emission of a photon.
probability of induced emission of a photon.
probability of induced absorption of a photon.
probability of relaxation from state 2 to state 1.
Finally, the induced processes occurring between states 3
and 2 under the influence of the laser field must also be
considered. The relevant rates are proportional to the difference in the population numbers N2 and N3 and to the
photon density p of the laser field. The effective cross-section σ for the emission or absorption of a photon arises as
a constant of proportionality:
dN 3
However, in Fig. 1.5 showing the principle, only the tranσ ⋅ c ⋅ p ⋅ ( N 2 N 3 ) induced rate.
sition probabilities that are significant for the pump and
dt
laser processes are indicated. All the designated levels are Therefore the variation in the population density of level
populated to some extent due to pumping. The extent to 3 with respect to time can be written as the sum of the
which each state is populated is given by the number Ni separate rates:
of Nd atoms which are in the relevant state i of excitation:
dN 3
state 1
state 2
state 3
state 4
N1
N2
N3
N4
Under the realistic assumption made in this example that
the Nd atoms only pass through the labelled excitation
steps, the sum of the population densities gives the Nd atoms which are available. The desired laser oscillation will,
however, only be achieved if an adequate population inversion can be established between states 3 and 2. The conditions under which laser emission occurs, together with
how the laser behaves, can be predicted by a model of the
so called rate equation model. Initially, the main interest
will be focused on continuous laser operation.
1.2.1Rate equation model for four levels
dt
σ ⋅ c ⋅ p ⋅( N 2
N 3 ) Γ ⋅ N 3 + Wp ⋅ N1
Furthermore, the assumption is made that the transition
from state 2 to state 1 is also so fast that only very few of
the particles accumulate in state N2 that means N2 = 0 and
the total number N0 of Nd atoms is therefore:
N 0 = N1 + N 3
Since N0 is constant, also dN0/dt = 0 and dN1/dt becomes
-dN3/dt. Therefore the variation of the population density
N1 with respect to time is:
dN1
dt
σ ⋅ c ⋅ p ⋅( N 2
N3 ) + Γ ⋅ N3
Wp ⋅ N1
It is important for the later laser process to know how the
photon density on the laser transition 3 to 2 varies with
respect to time. With each „induced“ absorption process
a photon is annihilated and a photon is created with each
induced emission process.
The model describes the situation in a simple but exact
dp
manner. Each of the levels involved is regarded as a reserσ ⋅ c ⋅ p ⋅( N 2 N3 )
induced
i
voir to which or from which “particles” flow. The particles
dt
used in this picture represent the Nd YAG atoms related Once created, the photon density does not remain in a
to their corresponding state. Particle streams flowing to resonator, instead it reduces with the time constant τ , beph
the level are given a positive sign, those flowing away are cause photons are leaving at the mirrors of the resonators
Page 6
or are lost in other ways.
dp
l=
dt
p
τ ph
losses
in the resonator due to dispersion, absorption or refraction.
Pp is the pump power and Pth is the threshold pump power.
Above the threshold pump power Pth the output power of
the laser increases linearly with the pump power.
The complete variation of the photon density with respect
to time is:
σ ⋅ c ⋅ p ⋅( N3
rel. output power
dp
dt
1.0
p
τ ph
N2 )
For simplification the population inversion N3-N2 is designated as n . The variation of the population inversion with
respect to time is obtained by the following relations:
dn
dt
σ ⋅ c ⋅ p Γ ⋅ n + Wp ⋅ ( N 0
n)
p ⋅ (σ ⋅ c ⋅ n
0.6
0.4
0.2
0.0
( 1.1 )
Fig. 1.6: Laser output power as a function of the pump
power
and for the photon density:
dp
dt
0.8
1
).
τ ph
( 1.2 )
1.2.4Solution of the rate equations
The slope αS of the straight line (Fig. 1.6) is one of the most
important parameters of a laser and is termed the slope
efficiency.
αS
The differential equations ( 1.1 ) and ( 1.2 ) form a pair of
coupled conditional equations for the two unknown functions n(t) and p(t). The equations are non-linear because
they both contain the term pn. Analytical solutions are
not known and one has to rely on computerised solutions.
η⋅
E 32
T
⋅
E 41 T + L
( 1.4 )
The relation E32 / E42 is also known as the quantum efficiency. It gives the energy ratio of a laser photon to the
pump photon. For the Nd:YAG laser pumped by a diode
laser this is, for example, 810 nm / 1064 nm = 0,76.
The value η is the quantum yield, but unfortunately some1.2.5Steady-state solution
times both quantities are commonly named as the quanWhen the system is in the state of equilibrium, i.e. for tum efficiency. For the laser designer it is important to
steady-state laser operation, the values for dn/dt and dp/dt obtain the highest possible output at the highest possible
are equal to zero. In this case an expression for the popula- efficiency. Another important feature is that the transmistion inversion is obtained immediately:
sion T for the resonator output mirror must be selected as
large as possible according to equation ( 1.4 ).
N ⋅ Wp
σ ⋅ c ⋅ p + Wp + Γ
When the laser is operated below or just at the threshold,
no photon field is formed (p=0). In this case Wp << Γ and
the threshold inversion is given by:
n (p
0)
n0
N⋅
Wp
Γ
This equation states that in a four-level laser an inversion immediately is produced when it is pumped. This is
a particular advantage as opposed to other laser systems.
Unfortunately, neither the photon density nor the pumping
rate are directly accessible by measurement.
0.8
0.7
L = 0.05
0.6
rel. outputpower
n
0.4
L = 0.2
0.3
0.2
0.1
0.0
However, the photon density is coupled to an easily measured quantity, i.e. the power applied in the pumping process. If the relationships between the photon density p and
the corresponding intensity, as well as the resonator output
and loss characteristics are considered, the output power
Pa of a four-level laser can be obtained as:
L = 0.1
0.5
0.0
0.2
0.4
0.6
0.8
1.0
transmission T
Fig. 1.7: Laser output power versus the transmission T of
the output mirror and the losses L.
However, this has the consequence that the threshold
pump power Pth increases and the output power decreases
according to equation ( 1.3 ). A compromise between both
E
T
equations must therefore be found.
Pa η ⋅ 32 ⋅ Pp Pth ⋅
( 1.3 )
E 41
T+L
In practice the losses L depend on various parameters of
In this equation E32 signifies the energy difference be- the resonator including the quality of the laser rod, the abtween states 3 and 2 (laser wavelength). E41 is the energy sorption losses of the laser mirror etc., so that a mathematidifference between states 4 and 1 (pump wavelength), T is cal formula covering all effects would be too complicated.
the transmission of the output coupling mirror, L is the loss It has proven useful to measure the curve of Fig. 1.7 di-
(
)
Page 7
rectly at the laser to find the degree of output coupling. A
series of output mirrors with various transmission values
is used for this purpose.
1.2.6Time-dependent solution
The previous solutions described the situation where the
laser operates in a steady state. However, for practical operation of the laser, conditions in disturbed equilibrium
must also be considered. These kinds of disturbances occur when the intensity of the pump-light source change
or the laser resonator is slightly disturbed mechanically.
Large deviations from the steady state are particularly
important when they cause problems (e.g. spiking and hot
spots), but also when they lead to useful operating modes
(Q switching). Small deviations of the steady state with
δn<<n or δp<<p lead to damped harmonic oscillations of
n and p. Larger deviations produce undamped anharmonic
oscillations. In this case, power peaks (spiking) of such a
intensity may occur that the laser mirrors or the Nd:YAG
materials can be destroyed. However, if disturbances are
carried out in a controlled manner, these types of power
peaks, which extend up into the gigawatt region (!), can
be used to advantage. Computerised solutions must be
used in calculating the solutions to the rate equations for
these cases. In the following, these cases are therefore only
qualitatively described.
optical surfaces so that the laser might destroy itself during switch-on. This behaviour which will be observed in
the latter experiments indicates, that the Nd YAG crystal
can store energy.
1.2.8Q-switching
The quality of a resonator is the quotient of the resonance
frequency and the half width of the resonance curve. The
definition is the same as for oscillator circuits known from
electronics. A low Q figure for a resonator signifies high
losses and vice versa. In a laser resonator with low Q, a
high inversion can be produced without laser oscillation,
because the threshold is high. If the resonator Q is then
suddenly switched to a higher value, a high photon density
is formed and a large part of the inversion stored in the
laser-active material transfers into the photon field. This
is a similar process as discussed with the occurrence of
spiking. In contrast to spiking, here the laser threshold is
controlled by the insertion and removal of resonator losses.
The losses can be switched with an electro-optical switch
or by a saturable absorber, which transmission depends
on the irradiated intensity. In this experimental set-up a
Cr:YAG crystal is used as saturable absorber. The energy level from which the laser emission of 1064 nm starts
is the 4F3/2 state (Fig. 1.4) which has a mean lifetime of
app. 250 µsec. This means that it takes 250 µsec before
the intensity of the spontaneous emission decreases to 1/e
1.2.7Spiking
of its starting value, when the pumping field is switched
A large deviation from the steady state undoubtedly oc- off suddenly. This mean lifetime will be measured in a
curs when the laser is switched on or when the pump-light later experiment. For a laser system, this behaviour can be
source is switched on. Until the threshold pump power Pth exploited for the generation of short pulses with high peak
is reached, there are practically no photons present in the powers. Although the time dependent solution of the rate
equations can not be derived by analytical expressions one
resonator.
can obtain some simple derivations under the following assumptions. Because of the very fast increase of the photon
density the terms in Eq. ( 1.1 ) of the pumping rate and
the rate of spontaneous emission can be neglected. The
development in time of the population density (Eq.( 1.1 ))
simplifies with
1
σ ⋅ c ⋅ τ ph
n th
in this case to:
Fig. 1.8: Spiking of the Nd:YAG laser
When the population inversion reaches the threshold, a
photon field is formed. However, due to the resonator
propagation time, it takes a while until the photon density
reaches the steady state value. During this period the inversion, which rises linearly with time, increases above the
value of the threshold inversion. This in turn means a more
rapid increase in the photon density. This rise is so rapid
that the inversion falls to a value slightly below the threshold and the laser oscillation stops (Fig. 1.8). The process
starts again, but this time the laser is only slightly below
the threshold and the expected inversion overshoot is not
so large as before. In this manner the system approaches
the steady state. The first power spike (initial spike) can
reach a peak power of a factor 100 to 1000 higher than
the steady state power value. Spiking therefore can cause
serious problems and it can lead to the destruction of the
dn
=
dt
n p
⋅
n th τ ph
( 1.5 )
and the photon density to:
dp
dt
(
n
n th
1) ⋅
p
τ ph
( 1.6 )
The above equations can be solved for the build up of the
giant pulse when the population density n is nearly constant and corresponds approximately to the starting population ni. Then Eq. ( 1.6 ) becomes:
dp
p
n
1
⋅( i
τ ph n th
1) ⋅ dt
with the solution for the photon density:
p (t )
e
(
ni
t
1)⋅
τ ph
n th
Page 8
In accordance to the above solution the density of the photons rises exponentially with a time constant τ , which is
due ni/nth >> 1 considerably lower than the life time τph of
the photons inside the resonator. An additional remarkable
aspect in the development in time of the photon density is
the point where the inversion n(t) is decreased to nth. At
this moment dp/dt becomes 0 and therefore p = const. =
pmax.
Eq. ( 1.6 ) can now be written as:
p max
τ ph
1.0
The negative sign indicates that the inversion is decreasing
further. No additional laser photons will be produced. The
photon density reaches its maximum and decays now with
the time constant
L
c ⋅ (1 R )
τ ph
0.6
0.4
10
0.2
08
0.0
0
2
4
6
8
10
12
14
surplus inversion ni/nth
16
18
20
Fig. 1.10: Attainable peak value of the photon density relative to the initial inversion ni versus the surplus inversion ni
/ nth
0.6
0.4
180000
02
160000
00
0
50
100
150
200
250
300
time in nsec
Fig. 1.9: Calculated photon density for ni / nth = 10, a resonator length L of 100 mm and resonator losses of 1%.
The rising part of the curve corresponds to the surplus inversion and the falling one to the photon life time inside
the resonator The peak value of the photon density is given
by:
p max
n 
n th ⋅ ln  th  (n th
 n 
ni )
i
and the peak value of the intensity Imax by:
Peak intensity in W/cm
2
relative photon density
related to the lifetime of the photons inside the resonator.
0.8
photon density ni
dn
dt
ant pulse for the YAG laser. For the He Ne Laser with a
lifetime of the starting level of some nano seconds no increase of the laser power under Q-switch operation will be
obtained. In the case of the Nd:YAG laser with its lifetime
of approx. 250 µsec of the starting level the duration of
the giant pulse amounts 50 nsec, so that peak powers can
be achieved which are considerably higher than the pump
power itself. There are some techniques to realise the giant
pulse operation of a laser. But they all work with the same
fundamental operation: the laser oscillation is prevented
until a sufficient surplus inversion is attained.
140000
120000
100000
80000
60000
40000
20000
0
0
2
4
6
8
10
12
14
Surplus inversion ni/nth
16
18
20
Fig. 1.11: Attainable peak intensities of the cw Nd YAGLaser at T=1% in Q-switch mode
Of course it is also important that the releasing of the oscillation takes place sufficiently quick. In the ideal case
I max
nth=∞ for the blocked resonator and nth=min for the released
resonator. If for instance the threshold is varying slowly
The cross section of the stimulated emission σ for the tran- in time also n / n is reducing as well as the attainable
i
th
sition 3-2 (Fig. 1.5) amounts σ = 8.8 10-19 cm-2 and the peak power. The
switching
of the Q of the resonator can
photon energy hν1064 nm = 2 10-19 Joule.
be achieved with pure mechanical, electro-optical moduAt this point of the discussion it becomes clear why lasers, lator, acoustooptic modulator or by means of a saturable
whose starting energy level for the laser process has a low absorber.
lifetime, do not exhibit a considerable increase of the out- These different types of Q-switch devices are described in
put power under Q-switch operation. These lasers can only the following chapters.
achieve a small value of a surplus inversion ni / nth.
In the case of the CO2 laser one can obtain a slight increase
of the output power termed as super pulse instead of gi-
1 R
⋅ h ⋅ ν ⋅ c ⋅ p max
2
Page 9
Laser Resonator: Stability criterion
2.0Laser Resonator
leave the resonator by protruding beyond the edges of the
mirrors. For the plane parallel resonator (A), in which the
light beam is only reflected and not modified in shape, it
2.1Types of resonators
must be ensured that both plane parallel mirrors are adjusted
exactly parallel to one another. This type of resoThe most simple optical resonator, the Fabry-Perot resonanator
is
the most difficult to adjust and to maintain in a
tor, consists of a pair of plane or spherical mirrors located
correctly
adjusted condition. The spherical resonator (C)
opposite one another. They are centred to a common optiis
the
most
simple to adjust, but has the disadvantage that
cal axis and are aligned perpendicular to this axis
undesired transverse modes can easily start to oscillate.
This means that the laser power is split up over a number
of modes which are separated spatially from one another
A
and which cannot be focused to a common point as with
longitudinal modes.
L
R
2
B
The hemispherical resonator represents a satisfactory
compromise and the stability range for this type of laser is
determined in the following. First of all, the g parameters
are defined.
g i =1
L
R
1
R
2
C
Fig. 2.1: Types of resonator
L
Ri
g - Parameter
L is the mirror separation and R is the radius of curvature
of the laser mirror. The index i is 1 for the left mirror and 2
for the mirror on the right side. If the product g1g2 satisfies
the condition
Parameter G2=1-L/R2
0 ≤ g1 ⋅ g 2 ≤ 1
stability criterion
There are basically three different types of optical resonators:
then the resonator is optically stable and the light, once
plane parallel resonator
A
produced,
does not leave the resonator by passing over the
hemispherical resonator
B
edges of the mirror. Instead the light remains within an
spherical resonator
C
upper limit referred to a distance parallel to the optical
axis of the resonator. The stability diagram for the case
For lasers in the low to medium power range (1 mWof interest here is shown in Fig. 2.2 for the hemispherical
200W), the hemispherical resonator is mainly used. Its fearesonator.
tures include high output powers with relatively uncritical
mechanical adjustment.
3
Apart from other parameters, the output power depends
on how much of the laser-active material is used. In this
respect the terms pump volume and mode volume are used.
The pump volume is the volume of the active material
2
which is illuminated by the pump radiation. In contrast
instable region
the mode volume is the volume which the laser modes
fill within the laser-active material. By selecting the focusing, the pump radiation and the resonator shape the
1
designer can influence both quantities. In the optimum
case the pump volume should be a little larger than the
workingmode volume. The mode volume depends on which beam
range
parameters are chosen within the laser resonator. These
parameters are determined by the selection of the type of
0
0
1
2
3
resonator , the radius R of curvature and separation L of
Parameter G1=1-L/R1
the mirrors. However, it should be noted that within certain limits the separation L of the mirrors cannot be varied Fig. 2.2: Stability diagram
at will for a given radius R of curvature.
For this resonator g1=1, since R1 = ∞ (plane mirror). All
resonators are unstable above the limiting curve g1 g2 = 1
2.2Stability criterion
and are stable below this limit. Since g1 = 1, then with a
The range in which a resonator configuration exhibits any fixed R2 the distance of mirror 2 can only be changed from
kind of optical stability is found by the stability criterion. L=0 (g2=1) to L=R2 (g2=0). The distance that is actually
A resonator is optically stable if, after any number of re- adjusted within this range depends on the application for
flections, the light still remains in the resonator due to the which the laser is to be optimized for. The closer the resoimaging characteristics of the mirrors used and does not nator is operated to the stability limit, then the more sensiDr. Walter Luhs - Jan. 1999, revised 2003, 2009, 2011
Page 10
Laser Resonator: Resonator modes
tive it is to maladjustment, because even small changes in
separation can take the resonator into the unstable region
(e.g. thermal expansion). The mirror separation can be
decreased to prevent this problem, but the mode volume
is reduced and this in turn significantly affects the output
power of the laser.
2.3Resonator modes
2.3.1Longitudinal modes
a
n
A
a
resonator with a length L of e.g. 50 mm, the mode spacing δν amounts to 3 GHz. In principle there are a very
large number of modes which can fit into the resonator.
However, the laser-active material can only amplify a certain limited range of these modes. For the Nd-YAG laser
the wavelength at which the maximum gain occurs is 1064
nm.
The region in which amplification takes place is given by
the gain bandwidth (similar to the emission bandwidth). It
should therefore be expected that for a resonator length
of 50 mm with a mode interval of 3 GHz, 30 longitudinal
modes will start to oscillate and the laser emission should
consist of a combination of discrete frequencies.
inhomogeneous gain profil
n+1
B
L
Fig. 2.3: Standing waves in a resonator with plane parallel
mirrors. In the upper example five longitudinal modes fit
into the resonator of length L and in the lower one n=8
The light-wave is reflected at the mirrors and returns along
the same path. The electric field strength of the wave is
therefore zero at the mirrors. For a certain separation L of
the mirrors only waves can be formed which have a field
strength of zero at both mirrors. Obviously, this is possible
for a large number of waves for which an integer number
n of their half wavelengths λ/2 fit in the resonator Fig. 2.3.
The waves which fit into the resonator are termed oscillating modes or simply modes. If this integer number is n,
then all waves fit into the resonator for which
n ⋅an
L
is true. The next neighbouring mode fulfils the condition
(n +1)⋅ a n +1 = L
The separation between the wavelength of two neighbouring modes is 2an+1-2an. If λ is the wavelength, ν the frequency and c the velocity of the wave, then the following
applies with:
an =
resonator modes
Fig. 2.4: Inhomogeneous gain profile with shown resonator modes. Each individual mode has almost independent
operating conditions within the group
From the model of the rate equations we already know that
in the steady-state operation of the laser the inversion is
reduced until its threshold is reached and that the excess
pump photons are converted into laser light.
This also means that the gain reaches the value at which
the losses are just compensated. According to Fig. 2.5 this
would be a horizontal straight line touching the gain curve
at its maximum. In this pictures only one mode would be
able to oscillate, i.e. the one situated closest to the point
where the line and curve meet. Just this one mode would
take the complete inversion by itself in a "winner takes it
all" manner.
the winner takes it all
loss curve
gain curve
λn
c
and ν =
2
λ
δλ = λ a ( n +1)
or:
δν
λa (n) =
2⋅ L
n ⋅ (n + 1)
c
2⋅ L
The magnitude of δν is termed the mode spacing. For a
resonator modes
Fig. 2.5: Homogeneously broadened gain profile with shown
resonator modes
Page 11
Laser Resonator: Resonator modes
2.3.2Gain profile
ture as the mirrors themselves. The situation is drawn in
case A in which a radiation field has formed symmetrically
Laser materials which consists of atoms or molecules with about the optical axis. At the resonator output one can see
different properties behave in a different manner. This is a round Gaussian shaped intensity distribution. But it is
particularly noticeable in the situation with gas lasers, e.g. also possible for a radiation field to be set up at an angle
the He-Ne laser, in which the atoms, to a certain extent, to the resonator’s optical axis. In principle a multitude of
move freely in a discharge tube.
this type of radiation field can develop, because in all of
Following the Maxwell-Boltzmann distribution, there are these cases the radius of curvature for the radiation field
groups of atoms which have different velocities. These dif- at the mirrors is the same as that of the mirrors. At the
ferent groups represent their own classes and gain profiles. resonator output one can now observe intensity distribuThe whole gain profile is no longer homogeneous but is tions spatially separated and no longer symmetrical about
instead inhomogeneous. These types of laser principally the axis of radiation. Since these modes do not oscillate
oscillate on a number of modes (multimode operation).
in the direction of the optical axis (longitudinal) but are
However, experience has shown that there are no purely mainly transversal, these modes are termed transversal
homogeneous or inhomogeneous systems. Therefore, the modes. Owing to the large number of modes, a convention
gain profile of the Nd:YAG laser is mainly homogeneous, has been adopted in which the relevant modes are given a
it also has smaller inhomogeneous parts which lead to the universal designation:
Nd:YAG laser oscillating multimode. Optically pumped
TEMmnq
systems with homogeneous gain profiles are particularly
TEM stands for Transverse Electromagnetic Modes. The
susceptible to variations in pump power and to disturindices m, n and q are integer numbers which state the
bances in the resonator length due to vibration, noise, etc.
number of intensity spots minus one in the X axis (m) and
The gain characteristic of the system resonator/laser matethe number in the Y axis (n) which are observed. The basis
rial is modulated by these types of effects and additional
for this consideration is the fundamental mode TEM00q
modes can occur. This effect is in fact exploited to genwhich produces just a round spot. In the example in Fig.
erate modes which are coupled together. This method of
2.6 (B) the designation is:
operation is termed mode locking due to gain modulation.
TEM 01q
Very short laser pulses in the picosecond region can be
The
number
q
states
how
many nodal points the standing
produced using this type of operating.
wave in the resonator has. This number does not have any
significance for the user of the laser and is therefore gener2.3.3Transverse modes
ally omitted.
For the sake of simplicity, the laser and resonator properties were discussed for an example of a plane parallel resonator. In practice this type of resonator is not used due to
its disadvantageous characteristics.
A
B
Fig. 2.6: A spherical resonator with oscillation in the fundamental TEM00q (A) and a transverse mode TEM01q (B)
The hemispherical resonator has become very popular,
since it exploits in a special manner the desired mode characteristics of the plane parallel resonator and the advantages of adjustment associated with the spherical resonator. However, a disadvantage accompanies this advantage.
Whereas almost exclusively longitudinal modes excite in
the plane parallel resonator, transversal modes can also
arise in spherical resonators. This effect is shown in Fig.
2.6. In contrast to Fig. 2.3 showing the standing waves in
the diagram of the electric field of the laser beam, here the
geometrical shape of the beam within the resonator is illustrated. When the laser operates in the steady state, the
wave-fronts at the mirrors have the same radius of curvaDr. Walter Luhs - Jan. 1999, revised 2003, 2009, 2011
Page 12
The pump-light source: Diode Laser
3.0The pump-light source
distribution of the laser beam is defined by the laser medium and not by the resonator as for normal lasers.
3.1Diode Laser
plied forward voltage (B) . The active zone contains both
electrons and holes which produce a photon when recombining.
High power Nd:YAG lasers are still mainly pumped with
discharge lamps. Commercially available laser systems
can output up to 2,000 watts of continuous-waves laser
p
n
power. If one bears in mind that the overall efficiency
of the Nd:YAG laser is about 1 - 2%, then the discharge
lamps must have a light output power of approximately A
100 - 200 kW.
From the light produced, only 2,000 W is converted into
laser power and the rest appears as heat which must be
extracted using complicated cooling systems.
The reason for this “poor” efficiency is that the light produced by the discharge lamps has a broad spectral distribuactive zone
tion and the Nd-YAG crystal can only accept the offered
light in a number of narrow absorption bands.
Up to the present time it has not been possible, in spite
of complex and intensive research, to develop discharge
lamps which have an emission tuned to the absorption
created photon
bands of the Nd:YAG crystal. Along these lines, the gas, B
which is excited in the discharge lamps, has been doped
with additives to produce preferred spectral emissions.
Fig. 3.1: pn junction without applied voltage (A), with ap-
Laser diodes do not have this disadvantage since they emit
intensive laser light in a narrow spectral range of only a
few nano meters. The wavelength of diode emission therefore matches an absorption band of the Nd:YAG crystal
very well. It is possible to achieve efficiencies of 50-80%
in this manner. However, there are not at present any laser
diodes available with output powers greater then 10 W. On
account of the attractive features of laser diodes which, in
contrast to discharge lamps, do not require any heavy duty
power supplies for high voltages (approx. 1000 V), intensive research has started to manufacture of high-power
laser diodes.
A further advantage of laser diodes is their very small size
which enables a large number of individual diodes to be integrated on one common chip. Rows of pump-light sources
with optical output powers into the kW region can be built
up with this type of laser diode arrays.
The laser diodes are a special class of lasers. They differ
from “conventional“ lasers in two points:
Firstly for the classical lasers the laser-active atoms (molecules or ions) are independent of one another and only
the same energy levels are used for the laser process. This
means in principle that in order to produce a population
inversion an infinite number of atoms can contribute
(Boltzmann statistics).
This is not the case with semiconductor lasers. Here a
defined energy level can only be occupied by two active
particles (electrons, Pauli principle). But in semiconductors, the wave functions of the individual atoms overlap to
form a common energy band and the extent to which the
level is occupied follows the Fermi Dirac statistics. When
considering the laser process, the transition between the
distribution of population in two energy bands instead of
two energy levels must be taken into account as for conventional lasers.
The second important difference concerns the propagation
of the laser light within the pn zone. The spatial intensity
These two points lead to the fact that the beam characteristics and the spectral properties of semiconductor lasers are
significantly different from those of conventional lasers:
For 1.) Laser diodes do not have any inherently defined
emission wavelength, because there are no two discrete
energy levels that are responsible for the laser process as
with traditional lasers, but rather an energy distribution of
electrons in energy bands.
For 2.) The production and guidance of the laser light
takes place in a very narrow space (pn layer). In contrast to
the conventional laser, the dimensions of the resonator are
about the same order of magnitude as the wavelength of
the laser beam. The spatial distribution of the laser beam
and the mode structure is defined by waveguides, whereas
the light is freely propagating within a resonator of a conventional type of laser. These two points influence the application of laser diodes.
AuZn contact
oxide layer
p-AlGaAs
n-AlGaAs
GaAs active zone
n-GaAs substrate
AuGe contact
p-AlGaAs
n-AlGaAs
Fig. 3.2: (GaAl)As semiconductor laser with double heterostructure and single stripe geometry
Before the laser beam from laser diodes can be used in the
usual manner, the strong divergence must be corrected by
sometimes complex optical systems.
Also, the corrected parallel beam does not have a round
cross-sectional shape, but is elliptical and sometimes almost rectangular. The corrections required to the beam of
a laser diode and the difficulties in obtaining the required
Page 13
wavelength of the laser beam can be changed by altering
the temperature. The wavelength increases with rising
temperature. This is due to the fact that with rising temperature the refractive index and the length of the active
zone, and therefore of the resonator, are increased.
Above a certain temperature the mode no longer fits into
the resonator and a different one oscillates for which the
conditions are now more favourable. Since the mode interval of the extremely short resonator (typically 300 µm)
is very large, the mode jump is about 0.3 nm. If the temperature is reduced, the laser jumps back in its wavelength
again. The return jump does not necessarily take place
into the initial mode.
laser threshold
Spontaneous
emission
"LED"
Laser power →
focusing characteristics with comparable power densities
mean that the expense involved in the optics obviates the
cost advantages of laser diodes.
Because of this, a way is sought of using the laser diode,
not as a primary high power laser, but instead, due to its
excellent other characteristics, as a pump-light source for
conventional laser systems. It should also be mentioned
that the above arguments are only relevant when considering laser systems with high powers for applications in
material processing. Without a doubt, laser diodes are excellent as primary lasers in communication techniques.
Fig. 3.2 shows a diagrammatic representation of a (GaAl)
As semiconductor laser with double heterostructure and
single stripe geometry. Charge carriers are injected into
the very thin (approx. 0,2 µm) active layer by applying a
voltage via the upper AuZn contact stripe which is only a
few µm wide. The active zone is embedded between two
hetero junction boundaries which act as barriers for the
charge carriers. If the flow of current is high enough, population inversion is formed in the active volume.
The laser beam leaves the active zone through the exit window. The crystal has such a high index of refraction that
the end surfaces have a sufficient degree of reflection so
that no further coating is required and they therefore act
as laser resonator mirrors.
There are a large number of types of diode lasers which
differ in their structure. Only multiple stripe laser diodes,
known as diode arrays, are available at present as highpower laser diodes with powers from 3 W upwards. Single
stripe diodes (Fig. 3.2) are currently limited in their output
power to about 3 W. Meanwhile output powers of up to kW
can be achieved by rows of a number of active zones on
one chip. The closer the separation between the stripes, the
lower the laser threshold will be.
Induced and
spontaneous
Emission
T1
T2 >T1
Injection current →
Fig. 3.4: Dependence of Laser power on the injection current with the temperature as a parameter
Wavelength →
Applications which rely on correct variable tuning of the
wavelength of the laser diode should be carried out in a region of the curve in Fig. 3.3 that is remote from any jumps.
With a sufficiently high gain “super modes” are created Similar behaviour is also observed when the injection curwhich correspond to one oscillation state for the coupled rent, and therefore the laser output power, is varied. Here
active zones. The beam profile of the super mode is dis- the change in wavelength arises mainly due to the increase
persed according to the extent of the stripe. A laser diode in the refractive index which in turn is due to the rise in
with a single stripe and an output power of 500 mW is used charge carrier density in the active zone. With higher output power the temperature also rises due to losses in the
for the Nd-YAG experimental laser.
form of heat in the active zone. The dependence of the
current, and therefore the output power, on the temperature is typical for semiconductors (Fig. 3.4). It is therefore
essential that in practical operation the beam power is continually monitored.
Temperature →
To achieve this, a photodiode is built into the laser housing
and the photodiode supplies a signal which is independent of temperature and which is proportional to the beam
power. This signal is used as input for a control circuit
with which the laser current is corrected and laser power
maintained constant.
Fig. 3.3: Emission wavelength in dependence of the laser
diode crystal temperature showing a hysteresis
A further characteristic of the diode laser is the strong
dependence of the laser wavelengths on the temperature
of the semiconductor laser (app. 0.25 nm/°K) and of the
injection current (app. 0.05 nm/mA). Users who need a
defined wavelength must maintain the temperature and
the injection current constant to the required values. The
Page 14
Injection Current
λ= const.
10
20
30
40
50
Temperature °C
Fig. 3.5: Injection current versus temperature for constant
wavelength, shown for two different wavelength
The wavelength of the laser diode can be expressed as a
function the two variables temperature T and injection
current I in the following equation:
λ (T, I) = λ (T0 , I0 ) + α T ⋅ (T T0 ) + α I ⋅ (I I0 )
2
2
n
n
+α T2 ⋅ (T T0 ) + α 2I ⋅ (I I0 )
+... + α Tn ⋅ (T T0 ) + α nI ⋅ (I I0 )
For practical considerations it is sufficient to use only the
linear terms of the above equation. This approximation is
good for δλ/λ > 10-6. For operation at constant wavelength,
λ(T,I) = const. = λc ,
the equation can be written as:
I
I0 +
1
(λ
αT
λ0 )+
αT
⋅ (T T0 )
αI
Once this curve is measured as it will be done later, the
important coefficients αT and αI can be determined.
Page 15
Second Harmonic Generation: Non-linear Optics
4.0Second Harmonic Generation
4.1Introduction
The saying “to shed light upon a matter” means, more or
less, to “illuminate” facts that are unclear. In 1644 Rene
Descartes published his metaphysical ideas on the essence
of light. Since then, people have been trying to shed light
upon “light” itself. According to his ideas, light consists
of scattered particles which have different speeds in different bodies. In 1667, R. Hooke claimed that this was all
nonsense. He was the first person who thought that light
consisted of quick oscillations. Huygens demonstrated
light ether in 1690.
In 1717 Newton proved that light has a transversal property. At that time, however, people could only imagine longitudinal waves, so Newton rejected the wave theory of light
completely. Newton’s authority over the subject prevented
the formulation of the wave theory of light for 100 years.
Unaffected by the dispute over the essence of light, James
Clerk Maxwell summarised the electrical and magnetic
appearances in a system of mathematical equations.
In 1856, when Kohlrausch and Weber found out through
measurements that the speed of electromagnetic waves
was the same as that of light, Maxwell came to the conclusion that light is an electromagnetic oscillation.
In 1888 Heinrich Hertz was able to give experimental
proof of electromagnetic waves. As can be easily imagined, due to the various interpretations on the nature of
light it took a long time for the electromagnetic theory to
be recognised as a basis for the sum of the physical experiences which could not be reduced any further.
But, as we now know, even this theory has its limitations.
It is possible to explain all appearances which occur in
light scattering using this theory. However, it fails in the
case of the emission and absorption of light.
Max Planck was able to solve the problems in this area
with his famous formula
E = hν.
According to this formula light possesses both qualities,
i.e. corpuscular as well as wavelike qualities. This paradoxical formula could finally be clarified through quantum
mechanics. There was a further change in classic optics
in the sixties of this century when lasers were discovered.
For the first time, light was subjected to an unusually high
intensity. People observed appearances such as the optical
frequency doubling which led to the formulation of nonlinear optics. In classic, i.e. linear optics, the scattering
of light in matter is described by both optical constants
dependent on the frequency, the refractive number n and
the absorption coefficient α. In present linear optics, these
variables are independent of the intensity of the light falling in. Reflection, refraction, scattering speed and weakening of light are therefore constants of the relevant medium and are not dependent on the light intensity. This
resulted in two important principles used everywhere in
optics: The superimposition principle (interference) and
maintaining the frequency. Both these conditions are only
valid in relatively small light intensities as can be obtained
from normal light sources. Neither the superimposition
principle nor the conservation of frequency apply to the
high intensities of lasers. Therefore, linear optics is only
specifically applicable to small light intensities. The following explanations concern optical frequency doubling
as it is carried out later with the radiation of a Nd YAGLaser.
4.2Interaction of light and matter
The appearances observed on the basis of the interaction
between light and matter can, in principle, be divided into
two groups.
A.
resonant phenomena
B.
non-resonant phenomena
In the case of resonant light the incoming radiated light
has an energy of E = hν, which corresponds to the energetic distance of a transition. Electrons of the atoms or molecules in their original position are transferred to E1 which
is in an excited state. In the example of non-resonant light,
the energy of the incoming radiated light is much smaller
than the energetic interval of the observed transition.
E
E
1
E
0
1
E=hν
E
0
resonant
non resonant
Fig. 4.1: Incoming radiated light is resonant to a transition
of the sample (left) and non-resonant (right) for a material
with another transition
There is still an interaction, in which, however, no transition of the electrons takes place. The interaction occurs
through the electromagnetic property of light together
with the electromagnetic property of matter. We now want
to promote the fact that our material does not conduct
electricity and that it is not magnetic. This is the case for
almost all optically transparent materials from which components suitable for optics, such as lenses, prisms, crystals
etc. are made. The interaction of light is reduced on the
electrical qualities by the selection of the material. These
qualities are described using Maxwell’s equations.
4.3Non-linear Optics
For a more vivid explanation it is helpful to imagine the
electrons as elastic particles bound to the nucleus.
An electrical field drives the electron out of its position of
rest and produces a dipole (or changes an already existing
dipole). If the electrical field is periodic , then the driving
out process (dielectric polarisation) will also be periodic
and the dipole will radiate with the same frequency as the
electrical field producing it but with a phase shift. This
phase shift is responsible for the fact that the phase speed
of light in matter is apparently slower than in a vacuum.
Page 16
Second Harmonic Generation: Phase matching
The photons do, in actual fact, always move at the speed of
light in a vacuum. There is, of course, a vacuum between
the atoms. The starting point for radiation in a medium
are the incoming radiated photons which for their part, excite the dielectric polarisation dipoles. These pass on the
radiation with a phase shift from dipole to dipole. If the
entering photons had to be marked in colour, very few of
the coloured ones would appear at the exit of the sample,
but a number of the ones that are not marked would exit.
When light is radiated into a transparent sample, the electrical field vector of light corresponds to the field E. If the
field intensity of the light radiated is increased, so is the
displacement of the electrons.
P χL ⋅ E
( 4.1 )
χL is the susceptibility. In the example shown in Fig. 4.3
the relation is not linear any longer, due to the high field
intensity, but
P
χ L ⋅ E + χ 2NL ⋅ E 2
( 4.2 )
.
Even higher terms occur as the field intensity increases
further. The cause for the frequency doubling is, however, the quadratic term. Since electromagnetic oscillation
is a result of the polarisation and it is not harmonic, light
waves will now occur with frequencies which are not contained in the exciting light.
4.4Frequency doubling
dielectric polarisation P
A Fourier analysis shows that in this case, a further frequency occurs apart from the fundamental oscillation.
linear curve
dielectric
polarisation P
0
field strength E
fundamental
wave
second harmonic
constant field
incoming wave
Fig. 4.2: Displacement of the electrons through an electrical field E
polarisation P
The displacement of the electrons depends not only on
the field intensity, but also on the “spring constant” of the
binding of the electron to the nucleus (susceptibility). If
the field strength or the susceptibility is large enough, the
displacement becomes so big that the connection between
the power and displacement, analogous to the diversion of
a spring over the area of proportionality (Hooke’s Law), is
not linear any longer.
nonlinear dielectric
polarisation
field strength E
low field strength
high field strength
0
0
0
Fig. 4.4: Occurrence of a wave with double frequency and
of a constant inner electrical field as well as the original
fundamental wave
If, for example, the radiation of the Nd:YAG laser is transmitted through a suitable doubling crystal, then a radiation
of 532 nm occurs apart from the fundamental wave at 1064
nm. This is a green radiation which is very clearly visible. If the intensity of the green radiation is high enough,
this can be transferred again with a further crystal in UV
radiation at 266 nm. Limits are only set by the availability of suitable doubling crystals. What properties should
this kind of crystal possess? As we can see in Fig. 4.3, the
susceptibility, i.e. the scale used for it is very large and an
electron can be displaced very easily. It is also important
that the electron is in a potential that is strongly anharmonic. This property can be found in some crystals which
do not allow the electrons to be displaced evenly in all directions because of their grid structure. Another important
factor is that the material should be such that the doubling
and incoming waves are not absorbed.
4.5Phase matching
An important question is, of course, the degree of effectivity ηSHG, for the production of the doubled radiation. It
depends on the variable of a non-linear constant C, on the
Fig. 4.3: Increased light field
used length L of the crystal used and on the intensity of the
The polarisation now contains frequencies which the driv- radiation of the fundamental wave.
ing field strength did not originally contain. In the example
shown in Fig. 4.2 the polarisation takes place in a linear
position to the field intensity according to:
incident light field
Page 17
ηSHG
P2 ν
Pν
C ⋅ Iν ⋅ L ⋅ F (δk )
2
rection of the impulses are maintained.
( 4.3 )
Since
P2ν denotes the power of the doubled radiation,
Pν the power of the fundamental wave,
Iν = Pν / A is the intensity of the fundamental wave, or the
power P with regard to the beam cross section A and L the
length of the crystal.
k
2⋅ π
in vacuum and k
λ
the phase matching condition
n (2⋅ ν)
n (ν )
2⋅ π
in matter,
n ( ν )⋅ λ
( 4.7 )
is a result of equation ( 4.6 ).
If the index of refraction n(2ν) of the doubled radiation is
the same as the index of refraction n(ν) of the fundamental
wave, the conversion occurs at the highest degree of efF (δk )
( 4.4 )
ficiency. This is not possible due to the normal dispersion.
Therefore birefractive crystals are used with which the
various refractive figures of the ordinary and extraordinary beams are exploited. If radiation occurs at a particuF(δk) is a function whose value and thus the degree of ef- lar angle with regard to the optical axis of the crystal, the
ficiency becomes maximum when δk = 0, i.e.
condition for phase matching can be fulfilled. This type
of matching is known as angle matching and is the type
δk k 2 ν k ν 0
which is mainly used.
k2ν is the wave number of the doubled wave and kν is the
10
wave number of the fundamental wave. When δk = 0 the
so called phase matching condition is fulfilled. This can
be easily concluded from the following observation. Let
08
us assume, for the time being, that the doubling crystal is
a black box and let us look at the conservation of energy
0.6
and impulses.
conversion efficiency
 L
sin δk ⋅ 

2
2
 L 
δk ⋅ 

2 
Photon 2
0.4
02
00
Photon 3
δk =0
phase matching
Fig. 4.6: Efficiency of the frequency doubling as a function
of phase matching according to Eq. ( 4.4 )
If radiation does not take place exactly at the matching
angle the degree of efficiency will be reduced considerably.
In the case of deviations within ± 2o (acceptance angle) we
can still observe a frequency doubling, for example, in the
case of KTP (Potassium Titanyl Phosphate). In practice the
Fig. 4.5: Formation of photon 3 from photon 1 and 2
crystals are fixed onto adjustable holders to achieve the
Photon 3 is formed out of the photons 1 and 2. Since pho- best possible degree of efficiency.
tons 1 and 2 are identical to the frequency doubling in this A further important aspect for the conversion efficiency
is, according to equation ( 4.3 ), the intensity of the fundacase, photon 3 possesses the energy
mental wave. Therefore two things must be observed when
setting up an experiment:
E 3 E 2 ν E1 + E 2 2 ⋅ h ⋅ ν
( 4.5 )
1. A maximum possible fundamental wave power should
be produced.
The conservation of the impulses is also the same since
2. The highest possible intensity should be created
there is no other mechanism that can take an impulse J.
through focusing.
This is why the frequency doubling is carried out inside
h
h
the resonator in a further experiment. The power of a laser
J3
⋅ k 2 ν J1 + J 2
⋅ kν
( 4.6 )
is
much higher within the resonator than outside. In addi2⋅ π
π
tion the doubling crystal is positioned in the beam waist. A
This means k2ν has to be equal to 2 kν for a correct energy KTP crystal is used in the experiment as a doubling crysand impulse balance. The frequency doubling occurs with tal. KTP stands for the compound K TiO PO4, Potassium
the highest degree of efficiency when the sum and the di- Titanyl Phosphate and is a positive (no > nao) crystal with
Photon 1
Page 18
two axes. This means that extraordinary refraction occurs
in different ways in the X and Y directions. The wave- Finally we will discuss the power of the frequency doubled
length dependency of the refractive index is given in the wave. Once the phase matching has been ensured, then
following equations for KTP and in Fig. 4.7.
C ⋅ L2
n x2 =2.10468 + 0.89342 λ2/[λ2 - 0.004438] - 0.01036 λ2
ny2 =2.14559 + 0.87629 λ2/[λ2 - 0.0485]
- 0.01173 λ2
n z2 =1.9446 + 1.3617 λ2/[λ2 -
- 0.01491 λ2
0.042]
P2 ν
A
⋅ Pν2
according to equation ( 4.3 ).
After this, the power P2ν of the frequency doubled wave increases quadratically with the power Pν of the fundamental
wave.
0.35 µm < λ < 4.5 µm
power of 2nd harmonic
2.0
inded of refraction →
Z-axis
1.9
Y-Axis
1.8
X-Axis
no(1064nm)
nao(532 nm)
1.7
400
600
800
1000
1200
1400
Wavelength [nm] →
Fig. 4.9: Quadratic relation between the power of the fundamental waves and that of the second harmonic.
Fig. 4.7: Dispersion of KTP
The condition n(2ν) = n(ν) must be fulfilled. This condition can be fulfilled because the value of the extraordinary
refractive index depends on the angle θ to the optical axis
Z of the KTP crystal whereas the ordinary index does
not. If the fundamental wave (laser beam at 1064 nm) is
beamed in at a particular angle then n(2ν) = n(ν). This angle is given by the intersection of the ellipse n2νao (θ) with
the circle nνo (Fig. 4.8).
optical Z-axis
ordinary index
of refraction
θ
Furthermore, it has been determined that there is no threshold in this process. Set-ups and arrangements for the efficient carrying out of the frequency doubling should therefore be selected. These should produce as large as possible
powerful fundamental. Since, for example, the Q-switch
operation leads to a considerable increase in power, this
method is explained in greater detail with the Nd -YAG
laser although it will not be carried out in this experiment.
We have not yet discussed the constant C in detail.
3/ 2
beam
C
µ 
d
2 ⋅  0  ⋅ ω 2 ⋅ ik3
 ε 
n
0
for the second
harmonic
extraordinary
index of refraction:
for the
fundamental
for the fundamental
for the second harmonic
Fig. 4.8: Index ellipsoid and phase matching for n(2ν) = n(ν)
According to geometric considerations based on Fig. 4.8
the angle θ can be calculated with the following equation.
sin 2 (θ)
power of fundamental wave
n o (ν) n o (2ν)
n ao2 (2ν) n o 2 (2ν)
2
2
The refractive indices in each case can be calculated from
the dispersion curves.
µ0 denotes the induction constant, ε0 the influence constant, w=2pn the angular frequency, n the index of refraction of
the crystal and dik are components of the susceptibility
tensor χ. Till now we have considered χ as a number, i.e.
the medium observed was isotropic. This is not valid for
the crystals used here. We will develop the ideas to the
extent that we will accept P and E as vectors.
P

(Px , Py , Pz ) and E (E x , E y , E z )

P


χ L ⋅ E + χ NL ⋅ E 2
This means that χL and χNL are now matrices. They are
known as tensors because they have the qualities which


make it possible to turn and stretch a vector E to P . We
are only interested in the non-linear part.
P NL =χNL [ Ex2 + Ey2 + Ez2 + 2EzEy + 2EzEx + 2ExEy ]
In terms of vectors and tensors the above equation would
be written as follows:
The KTP crystal is then cut in such a way that the laser
beam meets the crystal and the optical axis at the angle θ.
Page 19
Px 
 
Py 
 
 Pz 
 d11

 d 21

d
 31
d12
d13
d14
d15
d 22
d 23
d 24
d 255
d 32
d 33
d 34
d 35
 E 2x 


 E 2 

y

 E z2 


 2E z E y 


2
E
E
 z x 


2E x E y 
d16 

d 26 

d 36 
the fundamental wave. Apart from the phase matching we
have now taken into consideration the demands placed by
the tensor in non-linear polarisation. The condition for frequency doubling is, therefore, that the fundamental wave
is polarised in a defined manner. We have, until now, assumed that KTP is a crystal with only one axis. This was
to make the observations simpler. There is, however, very
little difference between both extraordinary refractive indices compared with the ordinary one (Fig. 4.7).
KTP is an orthorhombic crystal of the type mm 2 with the
following non-linear tensor χnl..
0

 0

 d 31
χ NL
0
0
d 32
0
0
d 33
0
d 24
d15
0
0
0
0
0
0





The following is the result for non-linear polarisation
based on the above equation:
Px (2ν)
2 ⋅ d15 ⋅ E z ⋅ E x
Py (2ν)
2 ⋅ d 24 ⋅ E z ⋅ E y
Pz (2ν)
d 31 ⋅ E 2x + d 32 ⋅ E 2y + d 33 ⋅ E 2z
According to Fig the phase condition has been adjusted because the fundamental wave runs as an ordinary beam in
the crystal at an angle θ to the optical axis. Therefore the
fundamental wave does no possess any component which
oscillates in the Z direction, i.e. Ez is zero. The second harmonic oscillates as an extraordinary wave out of necessity
since Px(2ν) and Py(2ν) are zero according to the above
equations.
⇒ Pz (2ν)
d 31 ⋅ E 2x + d 32 ⋅ E 2y
Now Pz(2ν) has to be maximised. Since E2x + E2y = E20 this
is fulfilled for
Pz (2ν, θ)
E 2x
E 2y
d 31 ⋅ E 2x ⋅ sin (θ) + d 32 ⋅ E 2y ⋅ cos (θ)
d 32 cos (θ)
⋅
d 31 sin (θ)
For KTP the values of the dik are:
=
6.5 10-12
d31
=
5.0 10-12
d32
= 13.7 10-12
d33
=
7.6 10-12
d24
=
6.1 10-12
d15
tan 2 (φ)
.
m/V
m/V
m/V
m/V
m/V
The azimuth angle φ of the incoming beam must be adjusted to the XY axis of the crystal to achieve a maximum
power of the second harmonic so that this relationship
of the amplitudes towards the crystal axis is adjusted. In
practice the crystal is turned around the entering axis of
Page 20
Q-switch: Optical Q-switch
5.0Q-switch
One distinguishes between two general kinds of
Q-switching. They are termed active if the moment of
switching can be determined by the operator and termed
passive when the moment of switching is controlled by the
laser process itself.
5.1Mechanical Q-switch
At the beginning of the laser technique one uses due to the
lack of better methods rotating apertures or laser mirrors
as shown in Fig. 5.1. In this set-up a wheel with laser mirrors ground on the wheel turns with an angular velocity ω.
If one of the mirrors is in exact position with respect to the
optical axis of the resonator the Q reaches its maximum.
In all other cases "the resonator is not a resonator"
rotating mirrors
ω
flashlamps
Nd:YAG rod
output coupler
Fig. 5.1: Mechanical Q-switch with rotating mirrors.
5.2Optical Q-switch
These more modern proven devices are until nowadays in
use.
high voltage
flashlamps
Wollaston’s prism
low voltage
Hf ON
lamps
Bragg’s cell
Hf OFF
lamps
Fig. 5.3: Acoustooptic Q-switch device
In this set-up a standing sound wave is introduced by applying a periodic electrical field to a crystal. The generated
sound waves are produce a periodic compression and decompression of the crystal which leads to a spatial variation of the index of refraction inside the crystal causing a
deflection of the passing light. When the applied electrical
field is switched off the light travels uninfluenced through
the crystal, the Q value now is high.
5.2.1Passive Q-switch
This technique is used in our later experiment. Its splendid
advantage lies in the fact that these devices are very cheap
compared to the active drivers. One only needs a material
whose absorption can be bleached under the irradiation of
the light from the stimulated emission in a laser resonator.
They only have one disadvantage, the timing of the switching can not controlled as the term “passive” indicates. The
effect of bleaching a material by light is a nonlinear optical
effect which is explained with the following simple two
level rate equation model.
In analogy to the previous rate equation model for four
levels the model for two levels is derived. For the level
E1 the change with time of the population density due to
absorption is given by:
dN1
dt
absorption
B12 ⋅ ρ (ν)⋅ N1
W12 ⋅ N1
The absorption process populates level 2:
Pockel’s cell
Fig. 5.2: Electro-optical Q-switch device
dN 2
dt
absorption
B12 ⋅ ρ (ν)⋅ N1
W12 ⋅ N1
Processes of spontaneous and induced emission are deThe Pockel’s cell consists of a crystal which becomes bi- populating level 2 by the rate:
refringent under the influence of an electrical field. At a
certain voltage this crystal acts as a quarter wave plate.
dN 2
A 21 ⋅ N 2
The light (stimulated emission) which has a defined state
spont .
dt
of polarisation due to the polarising beam splitter passes
through the cell. Depending on the applied voltage to the
dN 2
B21 ⋅ ρ (ν)⋅ N 2 W12 ⋅ N 2
induced
cell the light stays inside the resonator (high Q) or it leaves
dt
it via the beam splitter ( low Q ). This set-up is mainly
used for pulsed ( flash lamp pumped ) Nd:YAG laser. For In the stationary case where dN2 / dt = 0 the solution is:
cw YAG laser with only a few percent of output coupling
N2
W12
the damping of the above device is not sufficient for secure
=
suppressing of laser oscillation. For cw pumped systems
N1 W12 + A 21
the acoustooptic Q-switch of Fig. 5.3 is better suited.
Page 21
E2
N2
spontaneous emission
Absorption
induced emission
E1
N1
Fig. 5.4: Two level system
All kinds of two level systems can become transparent under the irradiation of light, but they differ in the necessary
amount of light intensity. There exist a number of suitable
saturable absorber, for instance dyes in solution or embedded in solids.
These kinds of absorber have the disadvantage that they
are not stable enough for the long times necessary for laser applications. As a suitable candidate Lithium Fluoride
(LiF2-) containing x-ray induced colour centres which are
responsible for the saturable absorption at 1064 nm has
been proven best in the past years. The density of the colour centres has to be chosen in such a way that the laser oscillation is prevented due to the initial absorption. A better
solution meanwhile is the Cr:YAG crystal which production is less complex since no x-ray treatment is required.
The increasing stimulated emission starts to bleach the
crystal to a value where the laser reaches its threshold. The
incipient laser light drives the crystal to higher transmission. Nearly the complete surplus inversion turns into the
photon field inside the resonator and a giant pulse is generated.
After the emission of the pulse the laser oscillation is interrupted due to the reduction of the inversion. The crystal
becomes absorptive and the process of bleaching starts
again.
absorbing
flashlamps
saturable absorber
flashlamps
Fig. 5.5: Passive Q-switch with saturable absorber
5.2.2Pockels’s cell
In the search for an optical switch which could be switched
on and off within a few nanoseconds (10 -9 sec), it soon became clear that no mechanical concepts could be considered. As electrical signals were switched on with corresponding speed, the search was on for an element without
any moving mechanical parts, which could follow a very
fast electrical field. Many different electro-optical effects which were known for a long time were considered.
However, a suitable optical material which on one hand
did not require a very high voltage potential and on the
other hand was optically very pure, was not found. These
qualities were essential because the optical switch was to
be used in a Laser resonator and thus its losses had to be
very small. Over and above that, extremely high maximum
outputs/peaks are produced in a Q-switch, which should
not damage the crystal. Already in 1893, Pockels had
shown that the addition of voltage on an optical medium,
changes its refractive index.
1.60
index of refraction
For high values of the pump rate ( W12 >> A21 ), N2 / N1
becomes app. 1. This means that the number of absorbed
photon becomes the same as the emitted ones. In this case
the medium is transparent.
1.55
1.50
1.45
1.40
0
1
2
3
4
DC voltage [kV]
5
6
Fig. 5.6: Linear electronic effect(Pockels effect)
n
n 0 ⋅ (1 + α ⋅ E )
n0 is the refractive index without adding outer voltage, α
is a material constant and E is the added voltage. However
till now we have not realised a switch, as a variation in
the refractive index does not cause any change in the amplitude of the intensity of light. When a double refracting
crystal is used as an optical medium, then the refractive index for the ordinary (OR) and the extra-ordinary axis(EO)
changes.
n OR
OR
n OR
⋅ E)
0 ⋅ (1 + α
n EO
n 0EO ⋅ (1 + α EO ⋅ E)
If the material constants αOR and αAO are different from
each other, then the difference in the refractive indices
dn(E) is dependent on the given voltage E.
dn (E )
n OR
n EO
transparent
Page 22
dn (E )
n 0EO + (n 0OR ⋅ α OR
n OR
0
n 0EO ⋅ α EO )⋅ E
First we will adhere to this result.
tween S and P components due to the different speeds of
the components in the crystal. Let’s refer back to the relation:
1 v
=
n c
index of refraction
1.60
where n is the refractive index, v is the speed of light in
the medium with the refractive index n and c the speed of
light in a vacuum. Because of the two different speeds of
light in a crystal, obviously two different refractive indices
are existing
1.55
dn
1.50
Up
1.45
1.40
vOR
1
λ OR
= OR =
c
n
λ
0
1
2
3
4
5
6
DC voltage [kV]
v EO
c
We use the relation:
Fig. 5.7: Linear electro-optical effect of a double refracting
material.
Let us recall the working of a quarter wave plate (also
called the lambda / 4 plate). Such a plate is made up of a
double refracting crystal, e.g. natural quartz. Such a crystal is optically not isotropic. Because of its natural crystalline growth, it has two optical directions which are caused
by the asymmetry of the crystal cells.
One can imagine the inner structure of a crystal to be
like that of a forest of well ordered antennae. There are as
many antennae which receive vertical polarised radiation.
These antennae are actually dipoles, which again radiate
the received radiation after a certain time.
The light wave falling on the crystal will be taken up by
the dipoles, one after the other and then radiated back. The
light wave slows down while passing through the crystal.
Because of the two types of the dipoles, two different
speeds of light are produced in the crystal.
If light with a polarisation of 45o is beamed into an “antennae forest”, then the intensity of the light is distributed
equally among the dipoles, because the light with the polarisation of 45o can be interpreted as an overlap of orthogonal components.
λ EO
λ
1
n EO
ν=
vOR
v EO
c vOR
=
=
=
λ λ OR λ OR λ EO
where v is the frequency of light, λ is the wavelength and
λm the wavelength in a medium, in which the speed of light
is v. Care is to be taken that the frequency is of a sustainable magnitude. The energy E of a light wave is:
E
h ⋅ν
and no energy from the light wave should be given up to
the medium. Note: The frequency of a light wave does not
change in a medium, only the wavelength, and therefore
the speed. This is valid as long as the medium is optically
linear. After leaving the crystal, both the partial waves
again move with the same speed, which is determined by
the refractive index of e.g. Air. But now they have a steady
phase shift ρ in respect to one another. As shown in Fig.
5.9, this phase shift ρ depends on the length d or thickness of the crystal. If one chooses the antennae, as also the
thickness of the crystal in such a way that the retardation
of the phases measures 90o or λ/4, then two waves which
run into each other are produced.
2
ρ
Amplitude
1
0
-1
Fig. 5.8: Double refractive crystal as a number of orthogonal dipoles.
At the exit from the crystal, a phase shift is observed beDr. Walter Luhs - Jan. 1999, revised 2003, 2009, 2011
-2
0
1
2
3
Multiple of
π
4
5
6
Page 23
Fig. 5.9: The two waves possess different speeds only inside
the crystal
shift measures λ/2, then one can be certain that the direction of polarisation of the falling light will be rotated
by
90o and with a phase shift of λ it will rotate by 180o.
They are polarised perpendicularly with respect to each
All
other phase shifts lead to elliptical polarised light. Let
o
other and they have a phase shift of 90 or λ/4 to each other
us
return
to the Pockels Cell. Here it is freely possible to
after leaving the crystal. The necessary thickness d here
change
the
value of dn by introducing a uniform voltage
can be calculated easily. For this we use the scalar repreand
thus
produce
phase shifts as required.
sentation of a light wave moving in a direction x.
The
polarisation
status
of the wave coming out can be elecThe wave number is k with
trically
directed.
If
we
place
a polarisation analyser behind
2π
the crystal, we can create such a polarisation direction
k
λ
with the help of an electrical field, so that either no light or
all the light passes the polariser. Furthermore, all the other
transmission factors that are desired can also be adjusted
ω the orbit frequency and t is the time
with the help of the voltage.
E
E 0 ⋅ sin (k ⋅ x + ω ⋅ t )
For the situation in the crystal, this means:
OR
E 0 ⋅ sin (k OR ⋅ x + ω ⋅ t )
E AO
E 0 ⋅ sin (k AO ⋅ x + ω ⋅ t )
E
left circular
For the phase shift ρ to correspond to λ/4, the following
must be valid.
(k OR k EO )⋅ d
 2π

 λ
OR
2π 
⋅ d
λ EO 
π
2
π
2
2π
π
⋅ (n OR n EO )⋅ d
λ
2
1
λ
d
⋅
4 n OR n EO
quarter wave plate
Fig. 5.10: Function of a quarter wave plate
If these intensity components are added as vectors, then
the resultant intensity vector rotates with the frequency of
the light beam around the axis of radiation. The light is
now polarised circular.
Depending on whether the quarter wave plate is met with
a irradiation angle of 45o or 135o right or left, circular polarised light is produced. If the thickness of the crystal is
chosen for example in such a way that the resultant phase
right circular
Fig. 5.11: Fig. 5.11: Circular polarised light
So now we possess an element that we can use as an optical switch. We will not be using a polariser in the following experiment, but we will dispose off the polarisation
dependent losses in the resonator.
For that, a Brewster window is introduced in the resonator, due to which the laser oscillates in a certain direction
of polarisation. As is already known from Fresnel’s equations, a light ray which enters a Brewster window parallel to the incidence level, experiences no reflection losses.
Opposed to that, the losses increase considerably for all
other polarisation directions due to reflection.
If we introduce a Pockels’s cell in the resonator, then we
can create another polarisation direction through the variation of voltage in the double refracting crystal, which produces such high losses at the Brewster windows, that the
laser cannot oscillate or the oscillation will be interrupted.
In this way, it is possible to control the quality of the resonator with the help of a Pockels’s cell.
Page 24
Set-Up and Performance of the Nd:YAG-Laser: The Modules
6.0Set-Up and Performance of the Nd:YAG-Laser
2
3
A
B
S
C
D
E
F
G
6.1The Modules
5
4
2
1
3
Module A: Diode laser 808 nm 500 mW
This module provides the laser light source (1) consisting of a temperature controlled laser diode. The maximum output power is 500 mW at 20° C. The laser
module is fixed into an adjustment holder (2) which is attached to a carrier (3) to
place the module onto the optical rail (S) having a length of 500 mm.
By means of a Peltier’s element the temperature of the laser diode can be changed
in a range from 15 to 40 °C. To control the injection current as well the temperature the diode laser head is connected to the ED-0020 controller.
The precision fine pitch screws (4 and 5) are used to align the optical axis of the
diode laser with respect to the mechanical axis of the set-up.
Module B: Collimator
The collimator consists of a three-lens system (1) with a short focal length
(f=8mm) and a large aperture in order to collimate divergent laser radiation emitting from the laser diode. The collimator (1) is mounted into a holder (2) which
is placed into the mounting plate (3).
2
1
3
1
2
Module C: Focusing unit
This unit is used to focus the collimated diode laser beam into the YAG rod
which is mounted in Module D. The lens has a focal length of 60 mm and is
mounted into a 25 mm click holder (1). The click holder is inserted into the
mounting plate where three spring loaded balls are keeping the holder precisely
in position.
Page 25
1
2
Module D: Laser mirror adjustment holder with Nd:YAG rod
Module D and E form the resonator of the Nd-YAG laser. The adjustable holders have the duty of adjusting the relevant resonator mirror so that the common
optical axis is aligned perpendicular to the mirrors. The plane-parallel YAG rod
(2), which is 5 mm long and has a diameter of 3 mm, is located in an exchangeable mount (1) in Module D (3). A coating, which is highly reflective at the laser
wavelength of 1064 nm, has been placed onto one end of the rod which also
forms the left resonator mirror. The coating is designed in such a way that the
maximum pump-light radiation can penetrate the highly reflective layer with
only 20% losses. The other end of the rod is coated with a high-quality antireflection layer for 1064 nm in order to keep the internal resonator losses as low as
possible. In addition the back side of the YAG rod is coated with a high reflective
layer for 532 nm in order to redirect the green light to the output of the resonator.
3
3
2
1
Module E: Laser mirror adjustment holder
This module contains the second resonator mirror (2). Also as with the laser rod,
it can be screwed out with its fixture (3) from the holder (1). The mirror has a
diameter of 1/2 inch and a radius of curvature of 100 mm. This mirror has a high
reflectivity (>99.98%) to keep as much of fundamental power within the resonator. The mirror is marked with “SHG100”.
1
2
Module F: Filter plate holder
The filter plate holder (1) comes with two filters. The colour filter RG1000 (2)
suppresses the pumping radiation of 808 nm. The second filter BG39 (3) allows
only the radiation of 532 nm to pass.
3
1
2
Module G: SiPIN Photodiode
This unit consists of a SiPIN photodiode accommodated in a 25 mm housing (1)
with attached cable. The detector is connected to the signal conditioner box ()
A target-screen can been mounted in this „click“ - mount (2) to check on the
optical axis during the basic adjustment. If required it can be inserted into the
mounting plate (2) instead of the photo detector.
ED-0060 Photodetector signal conditioning box
This device allows the connection of the photodetector to an oscilloscope. For
this purpose the photoelectric current which is proportional to the number of
incident photons needs to be converted into a voltage. This is done by the simple
however very effective by the circuit as shown on the left.
The circuit is driven by a 9 V battery which lasts almost one year under regular
operation. The impedance of the output can be adjusted from 50 Ω to 100 kΩ.
The “OFF” position still provides via an 1 MΩ shunt a signal with a very high
sensitivity which sometimes is useful.
For fast signals the lower shunt resistors are used. Rise times of 1 ns can be
measured in the 50 and 100 Ω position.
Page 26
Digital Laser Diode and Peltier’s Element Controller ED-020
This fully digital operating device controls the injection current as well as the
temperature of the diode laser head. By means of a one knob interaction all
parameters can be set and displayed. By turning the knob a specific menu item
is selected. Pressing the knob acts as enter key.
The diode laser head is connected via a multi-pin connector to the device. A
BNC jacket provides a synchronisation signal of the modulation frequency when
the diode laser is electronically switched on and off. The ED-0020 device provides an integrated USB interface which enables full remote control by an optional computer or laptop.
Specifications:
Injection current 1000 mA maximum, selectable in steps of 10 mA
Temperature
15 - 40 ° C in steps of 1°
Modulation
10 to 1000 Hz in steps of 10 Hz
Operating Voltage 12 VDC, by means of an extra wall plug power supply
Inputs
Diode laser connection
Outputs
Modulation signal as TTL trigger signal via BNC jacket
Dimensions
115 x 130 x 38 mm
12 V connector
pin 2.5 mm
7-pin diode laser
connector
Modulator signal
USB connector
On the left side of the ED-020 the connectors are located. A 12 V connection
with 2.5 mm pin is provided. A simple wall plug power supply is attached to here.
The diode laser connection requires a 7 pin connector and is suited for the DIMO
0.5 W laser head with integrated Peltier’s element and NTC temperature sensor.
However also other laser heads may be connected, please feel free to ask for
our support. A BNC jack is provided to deliver a monitor signal for the internal
modulator. This signal can be used as trigger. The USB connector is provided for
future expansions to control the unit by external software.
Laser ON / OFF
Turning the central knob highlights sequentially the selected item. To switch the
laser on, turn the knob until the “Laser” menu is highlighted as shown in the figure on the left side. Pressing the central knob shortly (less one second) switches
the laser on or off.
If a previous value of the current has been set and the laser is switched of, the
processor provides a soft shut down of the laser diode.
Injection current
Select the “Current” menu and press the central knob down for 2 seconds until
the “Current” item starts to blink. By turning the knob the current is selected
which is set to the laser diode from the processor with soft steps of 10 mA in
100 ms. This assures a safe and lifetime extending operation for the laser diode.
Pressing again the central knob leaves and closes the current menu.
Temperature
Select the “Temp.” menu and press the central knob down for 2 seconds until
the “Temp.” item starts to blink. By turning the knob the temperature set point
is selected. To take over the value the central knob must be pressed as long as
the menu item “Temp.” stops blinking. which is set to the laser diode from the
processor.
The temperature range can be set in from 15° up to 35° C. In highlighted mode
the value of the temperature is the temperature set point. In non highlighted
mode the actual temperature is displayed.
It may take a while before the system reaches the stable temperature.
Modulator
The injection current can be switched periodically on and off. In this mode the
output power of the laser diode is modulated as well. This is of interest, when
time dependant measurements shall be carried out. The modulation frequency
can be set from 0 to 1000 Hz. The duty cycle is fixed to 50%.
A monitor signal of this modulation signal is present as TTL signal at the BNC
jack as shown in the illustration of the left page.
Page 27
Set-UpandPerformanceoftheNd:YAG-Laser: Absorptionandwavelengthofthelaserdiode
6.2Experimental set-up
The object of the experiment is to set the semiconductor
laser into operation. The module A is positioned on the
optical rail and clamped.
The control unit is connected to its 12 Volt power supply.
The display of the controller turns on and is ready for operation.
The display of the diode laser controller shows the set value of the diode laser’s temperature as well as its current
temperature in degrees Celsius and the injection current
in mA. If the temperature is changed, then it takes a few
seconds until the set value has been stabilised at the laser
diode.
The laser output beam can be made visible with the IR
converter screen. The beam is so intense that it can even
be seen on a black surface. It can be seen that the diode
laser beam is divergent. The laser diode current is now
decreased to the lowest value and switched of to place the
collimator (Module B) onto the rail. The collimator has a
focal length of 8 mm. The focus is located about 1-2 mm
in front of the entry surface of the collimator. It is now
necessary to block off the emitted beam, so that it cannot leave the experimental area in an uncontrolled manner. This will be done by using the photo detector with
its adjustment aid target. The light from the laser diode is
almost parallel for a certain collimator position. The beam
spot should be centred to the crossed lines of the target. If
this is not the case, the beam path can be adjusted with the
adjustment screw on the module (A).
Consequently the diode laser is switched off again and the
focusing unit positioned on the rail. This unit contains a
biconvex lens with a focal length of 60 mm. It is later used
for focusing the diode laser beam into the Nd:YAG rod. It
is practical to set up the focusing module at a distance of
about 60 - 100 mm apart from the collimator. The focus
of the diode laser beam is produced at a distance of about
60 mm from the main plane of the biconvex lens. The
Nd:YAG rod should be positioned at this point, so that the
focus is located inside the rod. The position of the focus
can be found with a piece of white paper. It is noted before
the diode laser module is switched off again.
6.3Absorption and wavelength of the laser diode
6.3.1Adjusting a parallel beam by means of the collimator (module B)
A
B
laser spot
6.3.2Inserting the focusing unit (module C)
A
B
C
p o s it i o n
for Nd:YA
G rod
Page 28
Set-Up and Performance of the Nd:YAG-Laser: Absorption spectrum
6.3.3Inserting the YAG - rod
A
B
D
C
G
6.4Absorption spectrum
In the following experiment the dependence of the wavelength of the diode laser beam on the diode temperature
and the injection current is determined. Normally, these
types of measurements are carried out with a high resolution monochromator in which a grating is situated. The
grating is the element which is sensitive to the wavelength.
Another method is to use the well known absorption lines
of the Nd-YAG. The energy level diagram for Nd ions in
the YAG host crystal was shown in Fig. 1.4. According to
this diagram, there are four absorption transitions which
can be pumped with the laser diode used here. The maximum points of the absorption are located at:
1.
804.4 nm
2.
808.4 nm
3.
812.9 nm
4.
817.3 nm
given in sensitive ranges to ensure that no extraneous light
invalidates the measurement. At the start of the measurement the semiconductor laser module is switched on again.
The residual pump light passing through the YAG rod can
be observed with the converter screen. If the diode temperature is now changed, an increase or decrease in the intensity of the residual light can be observed which is caused
by the wavelength dependence of the semiconductor laser.
Once set, the level of injection current must be maintained
when carrying out the following measurement, because it
also affects the wavelength and the output power.
The measurement is taken, beginning with the lowest possible temperature. A period of a few minutes must expire
before the laser diode has cooled down to a constant value.
The measurements are then taken in suitable temperature
steps up to the maximum temperature.
1.0
808.4 nm
absorption (rel. units)
0.8
0.6
804.4 nm
812.9 nm
817.3 nm
0.4
0.2
0.0
10
20
30
40
50
60
diode laser temperature [°C]
Fig. 6.1: Absorption measurements for Nd-YAG versus the
wavelength of the pump light (laser diode temperature)
The adjustable holder with the YAG rod (Module D) is
used in addition to the existing set-up for the previous
measurement. The YAG rod should be positioned such
that the laser light illuminates the YAG rod centrally. The
supplied photodetector is positioned behind the YAG rod.
The distance should be chosen in such a way that the light
intensity does not saturate the detector. Attention must be
Page 29
Set-Up and Performance of the Nd:YAG-Laser: Laser diode output power
6.5Wavelength and temperature
dependence
A typical characteristic for the laser diode used is shown in
Fig. 6.3. This curve is enclosed with each experimental laser as individual data sheet. In practical operation it should
If the previous measurements are recorded, the spectrum be taken into account that return reflections from the lashowing dependence of absorption in temperature is ob- ser set-up can enter the laser diode and reach the monitor
tained for the Nd-YAG material. At least two or even three diode. This will be evaluated as laser power by the conpeaks should arise which can be allocated to the well trol circuit which will lead to a reduction in the injection
known central wavelengths. One peak is particularly no- current. Therefore, in the following laser experiments the
ticeable. The later laser experiments should be carried out pump-light beam should be slightly angled to the axis of
at this wavelength, because the pump efficiency η is the the laser resonator. The current indication is observed and
highest at this point. Here, it is necessary to be able to vary the adjustable holder for the laser mirror is tilted using the
the pump power without leaving the absorption peak, i.e. adjusting screw. If not necessary the power stabilisation
the power must be able to be changed without changing mode should be shut off.
600
the wavelength.
800
laserdiode power [mW]
500
Injection current [mA]
700
600
500
400
300
200
100
400
0
300
0
200
400
600
800
1000
injection current [mA]
26
28
30
32
34
Temperature [°C]
Fig. 6.2: Laser diode characteristic for operation at constant wavelength
An increase in the power increases the wavelength.
However, if the temperature is reduced by a certain amount,
then the wavelength remains constant. The temperature
value associated with each level of injection current must
be determined in another measurement. The same experimental set-up is used for this. A practical method is, to
first of all, set the temperature at which the absorption was
the highest or at which the transmission was the lowest.
This value should then be at the known central wavelength.
The injection current is then varied, changing the temperature so that maximum absorption occurs again. The pair
of values for temperature and injection current are noted
and drawn graphically. A straight-line operational characteristic is obtained for which the wavelength is constant
(Fig. 6.3).
6.6Laser diode output power
If an optical power meter is available, then the output power can be directly measured as a function of the injection
current. Due to the almost linear relationship of both quantities, the injection current can be used as a value for the
relative power.
Fig. 6.3: Typical laser power of the diode laser at 25°C.
The above experiments can be extended in a number of
ways or other experiments can be designed on the topic of
optical pumping with diode lasers. For example, the excitation spectrum of Nd-YAG can be measured if the supplied
cut-off filter RG1000 is placed in front of the photodiode.
The filter holder is intended for this purpose. Radiation
above about 1000 nm is let through the filter and the pump
light is therefore mainly suppressed. Induced radiation at
1064 nm and at 1384 nm is produced by the pump process.
Similarly still more fluorescent lines occur, but their spectral verification should be carried out with a monochromatic filter or a monochromator. The emission spectrum can
be measured similar to the manner in which the absorption spectrum was found. Without a monochromatic filter
an integral measurement of the fluorescence is obtained
in relationship to the pump wavelength. Deliberations regarding the wavelength stability of the diode laser can be
initiated be expanding on the knowledge obtained from
measurements on the calibration of the wavelength. Using
the absorption or transmission signal of the photodiode, a
control circuit can be formed which stabilises the laser diode wavelength exactly to the peak of a Nd-YAG absorption line. Estimates can be made about the long-term stability of this type of interesting application.
This is an advantage if the losses of individual optical
components in the beam path are unknown or if it would
be very difficult to measure them.
Page 30
Set-Up and Performance of the Nd:YAG-Laser: The 4F3/2 life-time measurement
6.7The 4F3/2 life-time measurement
The initial level for emission with a wavelength of 1064
nm is the 4F3/2 level, which compared to normal optical
transitions has a very long lifetime of about 250 µsec. This
means that 250 µsec pass before the intensity of the spontaneous emission has decayed to a value of 1/e of the initial
value.
If the Nd:YAG crystal is optically pumped periodically,
then the variation of the spontaneous emission with time
can be displayed on an oscilloscope. With the long lifetime
of 250 µsec this can be measured even with simple oscilloscopes.
For this experiment the internal modulator of the controller unit is switched on. The laser diode now is switched
on and off with an adjustable frequency which is set by
the associated knob on the front panel. The pump light is
focused into the laser rod with the focusing unit.
Fig. 6.4: Oscilloscope traces of pump power (upper) and
detector G (lower)
The RG 1000 filter is positioned close behind the YAG
rod to suppress the pump radiation that is not absorbed.
Fluorescent light passes through the filter to the photodetector G. The injection current output signal of the laser
diode and the output from the photodetector amplifier are
connected to a two channel oscilloscope. Fluorescent light
is still observed if the pump is switched off. At the point
at which the intensity of the fluorescent light has fallen to
1/e (0,37) of the initial intensity, this time is measured. It
corresponds to the mean life-time of the 4F3/2 level.
Page 31
Set-Up and Performance of the Nd:YAG-Laser: Laser operation
6.8The Nd-YAG laser set-up
It has been seen from the previous measurements that the
pump laser should be tuned to the strongest absorption line
at 808,4 nm (Fig. 6.1) and the value for the corresponding
temperature at a certain injection current is known (Fig.
6.2). The adjustable holder for the laser mirror (Module
E) is needed in addition to the existing set-up. The focus
of the pump beam must now be positioned such that it is
located centrally within the Nd-YAG laser rod. This can
be checked with the supplied adjustment aid target which
is inserted in front of the photo detector. The focus is located at the correct height when it meets the centre of the
crossed lines. Do this alignment with lowest possible power of the diode in order not to burn a hole into the target.
The sidewards alignment is carried out in a similar manner.
Module D is again used and positioned so that the focus is
located within the laser rod. The laser rod is then lined up
perpendicular to the pump radiation. This is done when
the gap between the adjustable and fixed plates of the laser
mirror adjustment holder is aligned parallel as shown in
the figure above. The second laser mirror holder (Module
E) is placed in position. The distance for the holder is provided by the section on optical stability. Initially it is sufficient to align the laser mirror perpendicular to the optical
axis just by sight. It is roughly lined up when the moveable
adjusting plate on the mirror holder is adjusted so that it is
parallel to the fixed base plate as shown in the figure above.
The diode laser is then switched on again and the current is
set to the maximum possible value.
6.10 Laser operation
and only the Nd-YAG laser radiation is passed through.
The laser output power is then optimised by adjusting the
resonator. One should ensure the correct resonator distance and the position of the focus in the laser rod which is
affected by the focusing lens. Further increases in power
can be achieved by slightly loosening the clamp and moving the lens holder. If no laser radiation occurs, then the
basic adjustment is made afresh. It has been found in practise that longer than 10 minutes should not be spent on adjusting the same element. The problem is then certain to be
somewhere else. A particularly sensitive way of adjusting
the laser is to use the photodetector with an oscilloscope
connected to it. The RG 1000 filter must be placed in front
of the detector and the oscilloscope must be switched to
the most sensitive range in the AC mode. A laser which is
just about to oscillate will already exhibit fluorescent light,
which is partially output from the output mirror. By moving the adjusting screw to and fro, the “flashing” of the
laser radiation will be noticed immediately on the oscilloscope as soon as it occurs. This method enables the tracing
of the point on the apparatus which is causing the problem.
6.10.1 Measurement of threshold and
output power of the Nd-YAG laser
50
40
laser power [mW]
6.9Laser resonator adjustment
30
20
Once the laser set-up has been adjusted, laser radiation
at 1064 nm should be produced. This radiation cannot be
seen with the eye and so the IR converter screen is used
10
for observation purposes. The screen is held in the beam
path on the output side of the resonator. When the laser
is excited, a whitish core is visible within the rest of the
0
0
50
100
150
200
250
pump radiation which appears on the screen as a clearly
pump
power
[mW]
formed pink-coloured rectangle. Depending on the state
of adjustment and the resonator distance, the laser beam Fig. 6.5: The laser output power in relationship to the
may appear spread out or ragged. To obtain a better check, pump power.
the plate holder with the RG 1000 filter is inserted into the Once the laser has been adjusted to the maximum output
path of the beam. The filter absorbs the pump radiation power, the slope efficiency and the laser threshold can be
Page 32
measured. The measured values provide information on
whether the system has an acceptable threshold value and
efficiency. The measurement of the output power is made
with an optical power meter at various pump powers. Here,
it should be ensured that the corresponding laser diode
temperature is set for each value of injection current. The
laser threshold can be found by reducing the pump power
until the laser just does not oscillate. At this point a fine
temperature adjustment and a re-adjustment of the resonator should be made.
6.10.3Display of transverse modes
Nd:YAG laser power [mW]
It will be found during the experiments that the laser has
the highest output power in multimode operation. The reason for this is that the active volume is larger than in the
fundamental longitudinal mode. The transverse modes can
be seen with the eye using the IR converter screen. If the
laser beam is expanded with a lens, then the structure of
the modes can be recognised. One can see a large number
of modes depending on the adjusted state and in one direction there are a particularly large number of modes. This
is
also the direction of the greatest expansion of the pump
6.10.2Output power at various pump
beam which is not round, but has a nearly rectangular
wavelengths
beam profile. This and the use of a low out coupling mirAccording to equation (10), the laser output power also de- ror are the reasons why the efficiency and output power
pends on the pump efficiency η. This can be checked in achieved do not reach the values which are theoretically
the following experiment. The injection current is set to a possible. The laser rod / resonator system, which is symfixed value and the pump wavelength is varied by chang- metrical about the axis of rotation, cannot fully convert all
ing the temperature. The output power is found for each the pump light provided. Better values are obtained if the
temperature value and displayed graphically.
pump-light beam is, for example, converted into an almost
round cross-sectional shape using a set-up of cylindrical
lenses. Later, when using the frequency doubling unit, the
60
transverse modes can be observed as visible light which is
much more impressive.
50
6.10.4Demonstration of Spiking
250 mW
40
For this experiment the internal modulator for the injection current of the laserdiode is switched on. The photodiode signal is displayed on the oscilloscope. Spiking can
be most impressively demonstrated if the laser is operated just above its threshold. The spiking curve becomes
more regular when fewer transversal modes oscillate. To
achieve this an aperture with variable diameter, which is
available as option, is placed into the resonator.
200 mW
30
20
10
0
0
10
20
30
40
50
60
laser diode temperature [°C]
Fig. 6.6: Laser power at various pump laser temperatures
(wavelengths). The laser output power peaks when the pump
wavelength is tuned to the corresponding absorption peak.
The curve as shown in Fig. 6.6 is obtained which has four
prominent peaks corresponding to the resonance frequency of the relevant absorption lines. Eq. ( 1.3 ) must be fulfilled for each point on the curve. The values for E41 are
known at the extreme values and a set of three equations
is available. Since the measurements were carried out at
the same values of T, L and Pp, the other quantities can
be obtained. If the same measurements are made with a
different pump power, a new set of equations is produced.
Since T is known, it is possible, to determine the losses
L. Those who are interested in mathematics can return to
the rate equations with the measurements from the range
of experiments. Comparing the measured values with calculated ones, it is possible to produce a computer model
of the Nd-YAG laser. The model can be further refined
through supplementary measurements to obtain “good”
parameters for the numerical solution, including the time
dependent solution of the rate equations. Dynamic processes such as spiking and Q-switching of the system can
be investigated with the time-dependent solution.
Fig. 6.7: The upper trace shows the photodetector signal
and the lower one the injection current
The diameter is set to suppress most of the transversal
modes. Those who are interested in obtaining a deeper
understanding of spiking and of the dynamic laser processes can perform a Fourier’s transformation on the curve.
It will be found that the periodicity of the spike pulses
depends on a number of parameters. These are mainly
the pump rate, the storage properties of the system, the
resonator propagation time, characteristic life-times and
relaxation rates. The above mentioned parameters can be
combined from these measurements on spiking and from
previous measurements to form a complete mathematical
model of the Nd-YAG laser.
Page 33
6.10.5Operation of the 1.3 µm Line
modes have to be controlled. The adjustable iris can be
set to a desired diameter in order to reduce the number of
Accordingly to the energy level diagram of Fig. 6.8 laser transverse modes. The smallest diameter is about 0,8 mm
operation can also carried out at the wavelength of 1.3 µm. and is sufficient to force the laser into TEM mode. The
00
For this purpose a YAG rod and the second laser mirror way the iris works can be impressive demonstrated
in conwith a high reflective coating for this wavelength is re- nection with the second harmonic generation.
quired. Since the previous used photodetector (Si type) is
not able to detect this wavelength it has to be exchanged
adjustable iris
against an Ge type. All these additional components are
supplied with the 1.3 µm options. Alignment and set-up
are identical to the operation at 1064 nm. Since the gain
of this transition is less than that of the previous discussed,
alignment is a little bit more complicated. But if one uses a
1 Watt diode laser no additional problems will occur.
4
F5/2
4
804.4 808.4 812.9
817.3
4
I9/2
1064 946
I 13/2
4
4
1322
F3/2
Fig. 6.9: Module Mode aperture
I11/2
absorption
emission
Fig. 6.8: Energy level diagram showing the 1.3 µm laser
transition.
6.10.6Mode Aperture and TEM00
Operation
In this case a great variety all kinds of transverse modes,
even the doughnut mode can be displayed. Furthermore
this element will be used to clear up the spiking or the generation of short pulses with the saturable absorber. Without
this element, the recorded signals will be the superimposition of a certain number of transverse modes.
When placing the iris inside the resonator one can control
the number of transverse modes.
This module will be used if the number of transverse
Fig. 6.10: Using the adjustable iris aperture to demonstrate the reduction of the transversal modes numbers.
Page 34
Second Harmonic Generation: Modules and Set-Up
7.0 Second Harmonic Generation
A
B
D
C
7.1 Modules and Set-Up
Y
ϕ
3
2
1
K
E
F
G
ficiency. It is also recommended to adjust the resonator
later on since beam bending can occur when the crystal
is placed in it. When the adjustment has been carried out
to a maximum exiting capacity for the “green” radiation,
the injection current of the laser diode is varied. This is for
measurement of the relation
P2ν = F(Pν).
θ
Fig. 7.1: Module K Frequency doubler
power of 2nd harmonic
X
The second harmonic at 532 nm of the fundamental wave
at 1064 nm is generated by means of a KTP crystal. The
crystal has a size of 2x2x6 mm and is fitted into a special
holder (1) to allow the cleaning from time to time. The
fitting is mounted into a holder which is inserted into the
5 axes adjustment unit. The module K is additionally supplied with a mounted laser mirror marked as SHG100. This
power of fundamental wave
mirror will be used instead of the output coupler (R1002%) and has a radius of curvature of 100 mm as well as a Fig. 7.2: Quadratic relationship between the power of the
fundamental and the second harmonic wave
high reflective coating of >99.98 % to keep as much power
as possible of the fundamental wave inside the resonator.
Also supplied to the module K is a BG39 filter, which suppresses all infrared radiation including the laser diode radiation and lets the green radiation with a transmission of
app. 60 % pass.
The intensity of the fundamental wave should be as high as
possible for an efficient frequency doubling. The module K
is now placed into the resonator. Please make sure that the
crystal with its end surfaces is preset, approximately vertically and centrally to the resonator axis (visual inspection).
According to how well adjusted the doubling crystal is, a
green radiation will come out of the exit of the laser.
We now start with the adjustment of the crystal which is
adjustable to all its axes in the holder. It can be rotated
on its axis, turned over and shifted into the XY direction.
The adjustments serve the purpose of achieving the best
phase matching and therefore the highest conversion efDr. Walter Luhs - Jan. 1999, revised 2003, 2009, 2011
Page 35
Passive Q-Switch: Modules and Set-Up
8.0Passive Q-Switch
8.1Modules and Set-Up
A
B
D
C
S
E
F
G
8.1.1Experimental arrangement for Q-switch operation
Y
ϕ
2
1
3
υ
X
saturable absorber
crystal
Fig. 8.2: Oscilloscope trace of Q-switch pulses
If no green beam is available insert the crystal when
aligned by eyes into the resonator. Connect the photodiode
to an oscilloscope which is switched to the highest sensitivity. Now tilt the adjustment screws of the adjustment
Fig. 8.1: Saturable absorber with mount and holder
holder or rotate the crystal with its mount. A flashing up of
The saturable absorber crystal (Cr:YAG) is set into a cen- the signal will be seen on the scope. If the modulator now
tring disk (1). The second disk keeps the crystal in position is switched on one can obtain a trace according to Fig. 8.2.
when the cap is screwed to the mount (3). The assemble d
crystal holder is inserted into the adjustment holder which
allows the precise adjustment in XY and υφ directions. In
the same way the crystal can be disassembled when cleaning becomes necessary.
For this experiment the saturable absorber (S) is placed
with its holder into the cavity. It must be ensured that the
crystal is preset perpendicular to the resonator axis.
If the experiment is supplied with the frequency doubling
option this can be done with the aid of the green beam. For
this purpose the crystal is placed at the output of the resonator and the crystal is aligned so that the back reflection
of the green beam is centred on itself.
Fig. 8.3: Modulation of the laser diode is switched on
By increasing the modulation frequency a single Q-switch
Page 36
Passive Q-Switch: Modules and Set-Up
pulse can be selected. With this combination of a passive
Q-switch device with the pump modulation a nearly active
Q-switch can be realised (Fig. 8.4).
00
380
360
power [mW]
3 0
320
300
280
260
2 0
220
200
500
520
5 0
560
580
600
620
6 0
660
680
700
injection current [mA]
Fig. 8.6: Diodelaser output as function of the injection current
Fig. 8.4: Nearly active Q-switch achieved with pump modulation
From Fig. 6.4 we measured the lifetime of the excited state
to approximately 250 µs. It means as well that the loading
(pumping) of the state takes time. This time depends on
the pump power. The higher the pump power is the faster
the excited state will be populated. However it makes no
sense to pump harder because of the lifetime for depopulation. This behaviour can be measured by taking the repetition rate of the pulses as function of the pump power which
is shown in Fig. 8.5.
,00
least square f t
im ting repetition rate
3,50
Repet tion rate [kHz]
3,00
2,50
2,00
1,50
1,00
250
270
290
310
330
350
370
390
Pump power [mW]
Fig. 8.5: Pulse repetition rate versus the pump power
It can be seen that a maximum repetition rate approximately 3.6 kHz results from the measured data. Theoretically 4
kHz as result from the lifetime of 250 µs should be possible. However this requires an ideal laser system which
exists only on paper.
The power taken for the graph of Fig. 8.5 is measured behind the focusing lens before it enters the Nd:YAG rod.
The relation of the injection current the absolute power is
given in Fig. 8.6.
Page 37
Active Q-switch: Modules and Set-Up
9.0Active Q-switch
9.1Modules and Set-Up
Trigger sig
nal from
ED-0020
controller
ON
OFF
Trigger
Input
Delay
High volta
ge Pocke
A
B
C
H.V.(kV)
ls’s cell dri
ver
D
E
P
F
G
9.1.1Description of the modules
2
ON
1
OFF
Trigger
Input
4
5
Delay
H.V.(kV)
Module Z “Pockel cell driver”
3
If the trigger input level (TTL) is high the output high voltage is on with the selected level set by the “H.V. (kV)” setting knob. Please note that the reading does not correspond
exactly to the displayed value.
2.0
Fig. 8.1: Module P “Pockels’s cell”
Output Voltage [kV]
The Pockel’s cell is provided with an rotatable Brewster
window (1). By loosening the screw with the supplied
1.5
tools the cap containing the window can be rotated. If a
maximum of output power is reached, the screw is fas1.0
tened again. The Brewster window (3) is covered by an
additional cap which prevents the damage of the window
as well as shielding Laser stray light coming from the win0.5
dow. Before removing of the cab the screw (5) must be
loosened. The Pockels’s cell is connected to the driver with
a special high voltage cable which forms an integral part
0.0
0
1
2
3
4
5
6
7
8
9
10
of the assembly and should not be exchanged. The trigger
Display
read
out
input of the driver is connected to the TTL output of the
frequency generator module. The falling edge of the TTLFig. 8.2: Output voltage versus display read out of the
signal switches off the high voltage and with that the status
Pockels’s cell driver
of polarisation.
Page 38
Active Q-switch: Active Q-switch alignment procedure
9.2Active Q-switch alignment procedure
Step I of the alignment procedure
First the Nd YAG laser is adjusted in the normal manner to
optimal output. It is helpful to work with the mirror SHG
100 because this shows almost no coupling losses, which
makes further adjustments considerably easier. The injection current of the laser diode is kept at a minimum to stop
the laser oscillation and permit the safe introduction of the
Module P (Pockels’s cell) into the resonator. Now the injection current is increased to the maximum value. Weak
laser oscillation should be seen on the oscilloscope. It is
necessary to switch on the internal modulator and to use
the injection current output signal at the second channel
of the oscilloscope as a reference signal. If no oscillation
is observed, the Brewster window is turned a little around
its axis position. Optimal output is adjusted with the right
laser mirror adjustment holder.
Final alignment step
After switching off the laser, the Pockels’s Cell is connected to the driver (Module Z). The trigger input is connected
to the TTL output of the frequency generator module and
the second channel of the oscilloscope. A trigger frequency value is selected in such a way that a flicker free picture of the TTL signal is seen on the oscilloscope. Before
switching on the high voltage driver, its setting should be
at the minimum. The injection current is again set at its
maximum. If this has not been done up to now, the channel
of the oscilloscope which shows the photodiode current is
set at the highest sensitivity. After adjusting of the high
voltage Q-switch pulses become clear.
NOTE: All the internal optical components of the resonator should be thoroughly cleaned.
If the laser in the Q-switch mode is optimised, well adjusted, with the right selection of the high voltage, then
in the following experiment the output coupling mirror R
100-2% can be used in place of the high reflecting SHG
100 mirror. For that, the injection current is again reduced
to the minimum. After inserting/ placing the mirror and
increasing the injection current to the maximum, the right
mirror adjustment holder is again optimised. Now, stronger Q - switch pulses should be recognisable.
Page 39
Active Q-switch: Extra-cavity second harmonic generation with Q-switched laser
9.3Intra-cavity second harmonic generation with Q-switched laser
A
B
C
D
P
E
F
G
This example shows the combination of the active q-switch
and the second harmonic generation. One can imagine that
also the peak power of single frequency doubled pulses
will dramatically increase. To verify this the BG39 filter
must be used to prevent infrared radiation to reach the photodetector and to allow the detection of only the green radiation. This filter is placed instead the RG1000 filter into
module F. It appears for the eye that the intensity seems to
be lower as in cw mode because the average power of the
green radiation is lower.
9.4Extra-cavity second harmonic generation with Q-switched laser
A
B
C
D
During this procedure, it is also possible to show the external frequency doubling with the help of the KTP crystal.
The KTP crystal with its holder is inserted into the laser
output beam. On a white sheet of paper, one can observe a
green point, the intensity of which can be visibly increased
by correct phase matching (rotation of the crystal in its
P
E
lens
f=60
G
K
holder) If one more focusing lens is available, then a considerable increase in the green radiation can be achieved.
For this, one uses the highest repetition rate of the high
voltage driver. Because of the high peak performance of
the Q- switch pulse, the external frequency doubling can
be demonstrated impressively
Page 40
Laser safety: Laser safety:
10.0 Laser safety
DANGER
INVISIBLE LASER RADIATION
AVOID DIRCET EXPOSURE
TO BEAM
DIODELASER
PEAK POWER 500 mW
WAVELENGTH 810 nm
CLASS 3b LASER PRODUCT
as possible.
d.) Eye protection is necessary if there is a possibility of
either direct or reflected radiation entering the eye or diffuse reflections can be seen which do not fulfil the conditions in c.).
e.) The entrances to supervised laser areas should be identified with the laser warning symbol
MZB means Maximum Permissible Radiation (Maximal
zulässige Bestrahlung) and it is defined in section 13 of
DIN/VDE 0837.
Special attention is drawn to point 12.4 of DIN VDE0837:
The Nd:YAG experimental laser is a laser which is only
suitable for laboratory applications.
With the individual modules in the assembled state, laser
radiation (semiconductor laser) can be produced at 805 nm
with a maximum power of 500 mW.
With the secondary Nd:YAG laser which is pumped by
the semiconductor laser, laser radiation at 1064 nm with a
maximum power of 100 mW can be produced.
The complete assembled laser is therefore a product which
exhibits the power characteristics of a Class IV laser.
Since the Nd:YAG experimental laser is a laser system
formed from combined nodular elements and can therefore be modified in a number of different ways, the operator of this system must ensure that the safety requirements
are met.
The manufacturer only provides a guarantee for the individual modules, but does not accept any responsibility for
cases of damage which arise due to the combination of the
modules. The user must observe the laser safety regulations, e.g. DIN VDE0837 or IEC 0837.
In these guidelines of February 1986 the following points
are listed for the operation of laser equipment in laboratories and places of work.
a.) The laser should only be operated in a supervised laser
area
b.) Special care should be taken to avoid unintentional reflections from mirrors
c.) Where possible the laser beam should terminate on a
material which scatters the light diffusely after the beam
has passed along its intended path. The colour and reflection properties of the material should enable the beam to
be diffused, so keeping the hazards due to reflection as low
Page 41
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